This PR generalizes a bunch of metadata structures.
Most importantly, it changes `SourceCache` to hold existentially quantified values:
```
data BackendSourceInfo =
forall b. Backend b => BackendSourceInfo (SourceInfo b)
type SourceCache = HashMap SourceName BackendSourceInfo
```
This changes a *lot* of things throughout the code. For now, all code using the schema cache explicitly casts sources to Postgres, meaning that if any non-Postgres `SourceInfo` makes it to the cache, it'll be ignored.
That means that after this PR is submitted, we can split work between two different aspects:
- creating `SourceInfo` for other backends
- handling those other sources down the line
GitOrigin-RevId: fb9ea00f32e840fc33c5467896fb1dfa5283ab42
This is an incremental PR towards https://github.com/hasura/graphql-engine/pull/5797
Co-authored-by: Anon Ray <ecthiender@users.noreply.github.com>
GitOrigin-RevId: a6cb8c239b2ff840a0095e78845f682af0e588a9
