### Description
This PR is the first of several PRs meant to introduce Generalized Joins. In this first PR, we add non-breaking changes to the Metadata types for DB-to-DB remote joins. Note that we are currently rejecting the new remote join format in order to keep folks from breaking their metadata (in case of a downgrade). These issues will be tackled (and JSON changes reverted) in subsequent PRs.
This PR also changes the way we construct the schema cache, and breaks the way we process sources in two steps: we first resolve each source and construct a cache of their tables' raw info, then in a second step we build the source output. This is so that we have access to the target source's tables when building db-to-db relationships.
### Notes
- this PR contains a few minor cleanups of the schema
- it also fixes a bug in how we do renames in remote schema relationships
- it introduces cross-source schema dependencies
https://github.com/hasura/graphql-engine-mono/pull/1727
Co-authored-by: Evie Ciobanu <1017953+eviefp@users.noreply.github.com>
GitOrigin-RevId: f625473077bc5fff5d941b70e9a116192bc1eb22
Fixes https://github.com/hasura/graphql-engine-mono/issues/712
Main point of interest: the `Hasura.SQL.Backend` module.
This PR creates an `Exists` type indexed by indexed type and packed constraint while hiding all of its complexity by not exporting the constructor.
Existential constructors/types which are no longer (directly) existential:
- [X] BackendSourceInfo :: BackendSourceInfo
- [x] BackendSourceMetadata :: BackendSourceMetadata
- [x] MOSourceObjId :: MetadatObjId
- [x] SOSourceObj :: SchemaObjId
- [x] RFDB :: RootField
- [x] LQP :: LiveQueryPlan
- [x] ExecutionStep :: ExecStepDB
This PR also removes ALL usages of `Typeable.cast` from our codebase. We still need to derive `Typeable` in a few places in order to be able to derive `Data` in one place. I have not dug deeper to see why this is needed.
GitOrigin-RevId: bb47e957192e4bb0af4c4116aee7bb92f7983445
