Initial subspace-gateway binary which does nothing

[teor2345]

This PR creates a subspace-gateway binary, but that binary doesn't actually do anything yet. The binary is based on the node and farmer implementations, with some simplifications.

I decided to call it subspace-gateway to keep the command name short, but not too generic.

Future PRs will add RPCs and DSN piece retrieval.

Code contributor checklist:

• I have read, understood and followed contributing guide

[nazar-pc]

Overall I think this should be closer to farmer rather than node in not relying on anything Substrate-specific from polkadot-sdk.

Also it is a bit of an odd one without anything actually implemented. I often start in a similar way, but open a PR with refactoring or some foundational work (like #3126 or #3128) once I have the next step done or close to it such that I know it'll work with higher probability.

I don't mind merging as is, just feels strange to have in the repo.