RPC communication with lazyledger-app #60

LazyLedger Data Availability client implementation will use RPC to communicate with lazyledger-app instance.
There were few not-working attempts to achieve this:

• using (legacy) JSONRPC: 995354c
• using gRPC: 153a7a6

It is possible, that some changes are also required in lazyledger-app to enable correct handling of PayForMessage message.
I tried to enable the gRPC endpoint.

Blocked by: celestiaorg/celestia-app#23