Externalize RPC snapshot testing

Implementers came to the conclusion that the best way to ensure API conformance would be to re-use existing [Forest RPC snapshots testing](https://docs.forest.chainsafe.io/developers/guides/rpc_test_snapshot).

Requirements:

- the RPC snapshot testing suite lives outside of Forest for clear separation and ease of contributing,
- Snapshots are publicly available and everyone can upload them (right now they are stored on ChainSafe's Digital Ocean bucket).
- Snapshots should also be easily verifiable. We might want to have some web RPC snapshot file explorer.

Node implementations will use the above for RPC regression tests.