Etherlink/Websocket: integrate `newHeads` websocket event to the evm node

Context

#7594 (closed)

Part. of %Websockets in EVM node.

Depends of !15561 (merged) and !15566 (merged).

This MR links all the previous preliminary ones (ws framework, encodings) to implement the first event of websockets which is newHeads.

Manually testing the MR

> dune exec etherlink/tezt/tests/main.exe -- --file evm_sequencer.ml evm rpc websocket http /threshold_encryption /dal  
> dune exec etherlink/tezt/tests/main.exe -- --file evm_sequencer.ml evm rpc websocket new_heads /threshold_encryption /dal

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