feat(db-*): add defaultValues to database schemas (#7368)

## Description

Prior to this change, the `defaultValue` for fields have only been used
in the application layer of Payload. With this change, you get the added
benefit of having the database columns get the default also. This is
especially helpful when adding new columns to postgres with existing
data to avoid needing to write complex migrations. In MongoDB this
change applies the default to the Mongoose model which is useful when
calling payload.db.create() directly.

This only works for statically defined values.

🙏 A big thanks to @r1tsuu for the feature and implementation idea as I
lifted some code from PR https://github.com/payloadcms/payload/pull/6983

- [x] I have read and understand the
[CONTRIBUTING.md](https://github.com/payloadcms/payload/blob/main/CONTRIBUTING.md)
document in this repository.

## Type of change

- [x] New feature (non-breaking change which adds functionality)
- [x] This change requires a documentation update

## Checklist:

- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] Existing test suite passes locally with my changes
- [x] I have made corresponding changes to the documentation

test/live-preview/fields/link.ts

@@ -137,7 +137,7 @@ const link: LinkType = ({ appearances, disableLabel = false, overrides = {} } =
     linkResult.fields.push({
       name: 'appearance',
       type: 'select',
-      defaultValue: 'default',
+      defaultValue: appearanceOptionsToUse[0].value,
       options: appearanceOptionsToUse,
       admin: {
         description: 'Choose how the link should be rendered.',