feat: collections can use custom database operations (#7675)

## Description
Adds option to override default database operations for a collection

```ts
import { CollectionConfig } from 'payload/types';

export const Collection: CollectionConfig = {
  slug: 'example-collection',
  // Database operations for this collection  
  db: {
    create: () => {},
    deleteMany: () => {},
    deleteOne: () => {},
    find: () => {},
    findOne: () => {},
    updateOne: () => {}
  },
  fields: [
    {
      name: 'someField',
      type: 'text',
    },
  ],
}
```
Co-authored-by: Dan Ribbens <dan.ribbens@gmail.com>

test/collections-db/config.ts

@@ -0,0 +1,68 @@
+import type { CollectionConfig } from '../../packages/payload/types'
+
+import { buildConfigWithDefaults } from '../buildConfigWithDefaults'
+import { devUser } from '../credentials'
+
+export const doc = {
+  id: -1,
+  customData: true,
+}
+export const docs = [doc]
+
+const collectionWithDb = (collectionSlug: string): CollectionConfig => {
+  return {
+    slug: collectionSlug,
+    db: {
+      // @ts-expect-error
+      create: () => {
+        return doc
+      },
+      // @ts-expect-error
+      deleteOne: () => {
+        return docs
+      },
+      // Only used in deleteUserPreferences on user collections
+      // @ts-expect-error
+      deleteMany: () => {
+        return docs
+      },
+      // @ts-expect-error
+      find: () => {
+        return { docs }
+      },
+      // @ts-expect-error
+      findOne: () => {
+        return doc
+      },
+      // @ts-expect-error
+      updateOne: () => {
+        return { ...doc, updated: true }
+      },
+    },
+    fields: [
+      {
+        name: 'name',
+        type: 'text',
+      },
+    ],
+  }
+}
+
+export const collectionSlug = 'collection-db'
+export default buildConfigWithDefaults({
+  // @ts-expect-error
+  collections: [collectionWithDb(collectionSlug)],
+  graphQL: {
+    schemaOutputFile: './test/collections-db/schema.graphql',
+  },
+
+  onInit: async (payload) => {
+    await payload.create({
+      collection: 'users',
+      data: {
+        email: devUser.email,
+        password: devUser.password,
+      },
+    })
+  },
+})

test/collections-db/int.spec.ts

@@ -0,0 +1,110 @@
+import payload from '../../packages/payload/src'
+import { devUser } from '../credentials'
+import { initPayloadTest } from '../helpers/configHelpers'
+import { collectionSlug } from './config'
+import { doc } from './config'
+
+require('isomorphic-fetch')
+
+let apiUrl
+let jwt
+
+const headers = {
+  'Content-Type': 'application/json',
+}
+const { email, password } = devUser
+
+describe('Collection Database Operations', () => {
+  // --__--__--__--__--__--__--__--__--__
+  // Boilerplate test setup/teardown
+  // --__--__--__--__--__--__--__--__--__
+  beforeAll(async () => {
+    const { serverURL } = await initPayloadTest({ __dirname, init: { local: false } })
+    apiUrl = `${serverURL}/api`
+
+    const response = await fetch(`${apiUrl}/users/login`, {
+      body: JSON.stringify({
+        email,
+        password,
+      }),
+      headers,
+      method: 'POST',
+    })
+
+    const data = await response.json()
+    jwt = data.token
+  })
+
+  afterAll(async () => {
+    if (typeof payload.db.destroy === 'function') {
+      await payload.db.destroy(payload)
+    }
+  })
+
+  // --__--__--__--__--__--__--__--__--__
+  // Local API
+  // --__--__--__--__--__--__--__--__--__
+
+  it('collection DB Create', async () => {
+    const result = await payload.create({
+      collection: collectionSlug,
+      data: {
+        id: doc.id,
+      },
+    })
+
+    expect(result.id).toEqual(doc.id)
+    expect(result.customData).toEqual(doc.customData)
+  })
+
+  it('collection DB Update', async () => {
+    const where = { id: { equals: doc.id } }
+    const result = await payload.update({
+      collection: collectionSlug,
+      where,
+      data: {
+        id: doc.id,
+      },
+    })
+
+    expect(result.docs[0].id).toEqual(doc.id)
+    expect(result.docs[0].customData).toEqual(doc.customData)
+    expect(result.docs[0].updated).toEqual(true)
+  })
+
+  it('collection DB Find', async () => {
+    const where = { id: { equals: doc.id } }
+    const result = await payload.find({
+      collection: collectionSlug,
+      where,
+    })
+
+    expect(result.docs[0].id).toEqual(doc.id)
+    expect(result.docs[0].customData).toEqual(doc.customData)
+  })
+
+  it('collection DB Find One', async () => {
+    const result = await payload.findByID({
+      collection: collectionSlug,
+      id: doc.id,
+    })
+
+    expect(result.id).toEqual(doc.id)
+    expect(result.customData).toEqual(doc.customData)
+  })
+
+  it('collection DB Delete', async () => {
+    const where = { id: { equals: doc.id } }
+
+    const result = await payload.delete({
+      collection: collectionSlug,
+      depth: 0,
+      user: devUser,
+      where,
+    })
+
+    expect(result.docs[0].id).toEqual(doc.id)
+    expect(result.docs[0].customData).toEqual(doc.customData)
+    expect(result.errors).toHaveLength(0)
+  })
+})