### Description
This PR cleans `processRemoteJoins` by splitting the code, introducing comments, and applied the same strategies than #3810 did. Most importantly, it introduces a new module `RemoteJoin.Source`, made to be very similar to `RemoteJoin.RemoteSchema`, that exposes the required tooling to make a join call to a source, which decluters `Join`. Furthermore, this PR uses the same "dependency injection" to make the core of `Join` free from IO: this opens the door to testing the join engine in the unit tests.
None of the functions were modified when moved from their old module to the new one, but there's no way to easily see this in a diff.
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/3894
GitOrigin-RevId: 1e7c43006f092326e061f9ba12674e207b628bef
