feat: allow users/plugins to modify and extend generated types for fields & config, add generated types for json field (#6984)

- Improves type for `jsonSchema` property of JSON field
- Adds type generation of JSON field with `jsonSchema`
- Adds `typescriptSchema` property to fields that allows you override
default field type generation by providing a JSON schema.
- Adds `typescript.schema` property in payload config, to allow for any
modifications of the type schemas

---------

Co-authored-by: Alessio Gravili <alessio@gravili.de>

docs/typescript/generating-types.mdx

@@ -61,16 +61,55 @@ You can specify where you want your types to be generated by adding a property t
 
 The above example places your types next to your Payload config itself as the file `generated-types.ts`.
 
+## Custom generated types
+
+Payload generates your types based on a JSON schema. You can extend that JSON schema, and thus the generated types, by passing a function to `typescript.schema`:
+
+```ts
+// payload.config.ts
+{
+  // ...
+  typescript: {
+    schema: [
+      ({ jsonSchema }) => {
+        // Modify the JSON schema here
+        jsonSchema.definitions.Test = {
+          type: 'object',
+          properties: {
+            title: { type: 'string' },
+            content: { type: 'string' },
+          },
+          required: ['title', 'content'],
+        }
+        return jsonSchema
+      },
+    ]
+  }
+}
+
+// This will generate the following type in your payload-types.ts:
+
+export interface Test {
+  title: string;
+  content: string;
+  [k: string]: unknown;
+}
+```
+
+This function takes the existing JSON schema as an argument and returns the modified JSON schema. It can be useful for plugins that wish to generate their own types.
+
 ## Example Usage
 
 For example, let's look at the following simple Payload config:
 
 ```ts
+import type { Config } from 'payload'
+
 const config: Config = {
   serverURL: process.env.PAYLOAD_PUBLIC_SERVER_URL,
   admin: {
     user: 'users',
-  }
+  },
   collections: [
     {
       slug: 'users',