Data stream improvements

[pmikolajczyk41]

This PR cleans up the API and internals of the data streaming protocol for DA, extracted in #3603 and #3625.

1. Abstract signing and verifying messages within the protocol. Instead of using das.SignatureVerifier and das.applyDasSigner we can now use pure signing utilities from signature package. This allows avoiding overhead for building Merkle tree over each payload.
2. Moved both sender and receiver parts to a separate submodule (hiding internals, enhancing modularity, cleaning up dependencies).
3. Added basic tests for the protocol.
4. Made sender generic over the return type (i.e., return whatever server returns, stop depending on the concrete StoreResult from DA RPC server)
5. Removed rpcClientSendChunkSuccessGauge and rpcClientSendChunkFailureGauge from the RPC client - these were low-level-implementation-details-based metrics that would have to be strangely passed to the DataStreamer
6. Minor refactoring, renaming, fixed few typos and license headers.

part of: NIT-3846