### Description
This PR rewrites OpenAPI to be more idiomatic. Some noteworthy changes:
- we accumulate all required information during the Analyze phase, to avoid having to do a single lookup in the schema cache during the OpenAPI generation phase (we now only need the schema cache as input to run the analysis)
- we no longer build intermediary endpoint information and aggregate it, we directly build the the `PathItem` for each endpoint; additionally, that means we no longer have to assume that different methods have the same metadata
- we no longer have to first declare types, then craft references: we do everything in one step
- we now properly deal with nullability by treating "typeName" and "typeName!" as different
- we add a bunch of additional fields in the generated "schema", such as title
- we do now support enum values in both input and output positions
- checking whether the request body is required is now performed on the fly rather than by introspecting the generated schema
- the methods in the file are sorted by topic
### Controversial point
However, this PR creates some additional complexity, that we might not want to keep. The main complexity is _knot-tying_: to avoid lookups when generating the OpenAPI, it builds an actual graph of input types, which means that we need something similar to (but simpler than) `MonadSchema`, to avoid infinite recursions when analyzing the input types of a query. To do this, this PR introduces `CircularT`, a lesser `SchemaT` that aims at avoiding ever having to reinvent this particular wheel ever again.
### Remaining work
- [x] fix existing tests (they are all failing due to some of the schema changes)
- [ ] add tests to cover the new features:
- [x] tests for `CircularT`
- [ ] tests for enums in output schemas
- [x] extract / document `CircularT` if we wish to keep it
- [x] add more comments to `OpenAPI`
- [x] have a second look at `buildVariableSchema`
- [x] fix all missing diagnostics in `Analyze`
- [x] add a Changelog entry?
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/4654
Co-authored-by: David Overton <7734777+dmoverton@users.noreply.github.com>
GitOrigin-RevId: f4a9191f22dfcc1dccefd6a52f5c586b6ad17172
