### Description
This PR removes the need for the `SourceCache` when building the schema for the actions. To do so, it changes the way we represent custom types in the source cache. Instead of trying to reuse the same `ObjectTypeDefinition` and `TypeRelationship`. we now have separate `AnnotatedObjectType` and `AnnotatedRelationship`. When building them, at schema cache building time, we persist all the relevant source information, so that it's all available at schema building time.
This PR makes no attempt at re-using `RemoteRelationship` primitives, to avoid having to change the way async action queries are executed, and to avoid having to make complicated changes to how we parse and represent those relationships.
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/4813
GitOrigin-RevId: 3cc65c5a043c8d3da5f7214eed40c558c4349327
### Description
This PR is a first step in a series of cleanups of action relationships. This first step does not contain any behavioral change, and it simply reorganizes / prunes / rearranges / documents the code. Mainly:
- it divides some files in RQL.Types between metadata types, schema cache types, execution types;
- it renames some types for consistency;
- it minimizes exports and prunes unnecessary types;
- it moves some types in places where they make more sense;
- it replaces uses of `DMap BackendTag` with `BackendMap`.
Most of the "movement" within files re-organizes declarations in a "top-down" fashion, by moving all TH splices to the end of the file, which avoids order or declarations mattering.
### Optional list types
One main type change this PR makes is a replacement of variant list types in `CustomTypes.hs`; we had `Maybe [a]`, or sometimes `Maybe (NonEmpty a)`. This PR harmonizes all of them to `[a]`, as most of the code would use them as such, by doing `fromMaybe []` or `maybe [] toList`.
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/4613
GitOrigin-RevId: bc624e10df587eba862ff27a5e8021b32d0d78a2
A very minor cleanup (came out of documenting the architecture of actions). Does what's mentioned in the title.
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/4451
GitOrigin-RevId: d480ff438256df468df65b43d15f92a30b14b997
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## Description ✍️
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Add some documentation on `ActionFieldG` type.
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/3945
GitOrigin-RevId: d9543ed7a8fe3ccfe9f5267c3a2ac71fb040f4db
