Add an initial gateway RPC crate

[teor2345]

This PR adds an initial object fetching RPC as a separate crate. Having a separate crate allows RPC clients to depend on the RPC crate, rather than the whole gateway binary.

This PR is independent of the gateway binary and gateway DSN PRs.
It will be integrated into the gateway binary in an upcoming PR.

Code contributor checklist:

• I have read, understood and followed contributing guide

[nazar-pc]

Have you considered making subspace-gateway both binary and library like farmer is?

[teor2345]

Have you considered making subspace-gateway both binary and library like farmer is?

That’s possible, but I think it’s a decision we can make later.

A Rust RPC client would just want to pull in the RPC definitions in this subspace-gateway-rpc crate, and its minimal dependencies. (And if that’s still too much, we can add a feature later to drop the dependency on the object fetcher and its crate.)

If we make subspace-gateway into a binary and library, then it could be used to launch a DSN connection, create the object fetcher, and then launch the RPC server in this crate. Which is a lot more code and dependencies.