Fixes https://github.com/payloadcms/payload/issues/8413 and https://github.com/payloadcms/payload/issues/6460 - Builds indexes for relationships by default in the SQL schema - Fixes `unique: true` handling with Postgres / SQLite for every type of relationships (non-polymorphic. hasMany, polymorphic, polymorphic hasMany) _note_: disables unique for nested to arrays / blocks relationships in the `_rels` table. - adds tests 2.0 PR tha ports only indexes creation https://github.com/payloadcms/payload/pull/8446, because `unique: true` could be a breaking change if someone has incosistent unique data in the database.