support schema contracts for single schema projects

[n1ru4l]

It is the same as #2662 but for single schema projects

To support this we need the following.

1. Light "Specification" of @tag for single schema projects (maybe opt-in via settings, or should it be done similarly to federation schemas with a @link directive on the Schema object?)
2. Schema filtering logic (we can probably re-use what we already have for federation subgraph filtering)
3. A Proxy gateway for serving the contract schema in front of the "full graph"

The proxy gateway is optional from the Hive perspective, though we should have the full usage path covered for pleasant usage. For the latter conductor will do. See the-guild-org/conductor#436

[tandrup]

We have build our own custom schema transform to do basically schema contracts in our schema stitching based setup.
But would be nice to have it supported in GraphQL Hive. #3593 was partly because of these transformations.