support schema contracts for single schema projects

It is the same as https://github.com/kamilkisiela/graphql-hive/issues/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](https://github.com/the-guild-org/conductor) will do. See https://github.com/the-guild-org/conductor/issues/436