[EVM/DSN] Single node echo consensus

Depends on !11652

What

This is a single node "echo" consensus that aggregates incoming transactions in batches, committing every 500 ms.

Why

A next step towards the fully-fledged consensus on top of an authenticated p2p network,

How

Implementation details:

• Actor model on top of tokio library
• Protocol is abstracted from particular storage, network (todo), configuration (todo) backends
• There is a local protocol client that implements certain public APIs
• gRPC server handles remote user requests, spawning local protocol clients on-demand
• DSN node spawns protocol runner and gRPC server and handle graceful shutdown

Manually testing the MR

Run tests:

cargo test

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR