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

Context

Depends on !15899 (merged).

Fix #7641 (closed).

Manually testing the MR

dune exec etherlink/tezt/tests/main.exe -- --file evm_sequencer.ml evm rpc websocket new_pending_transactions /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