feature request: simulate celestia node client write path

[tuxcanfly]

Problem

OP stack integration uses the celestia node client which writes to the core gRPC service. Currently localestia mocks the blob submit method of node API which is not be being used by the client.

Suggestion

It would be a good enhancement for localcestia to mock the core gRPC service and update the redis backend so that the client can be tested against it in both read and write paths.