EVM/Node: open stream with L2 to monitor heads

Context

Attempt to close #6438 (closed)

It opens a stream with the rollup node to get notified on new head. Therefore, the proxy will inject its operations at the beginning of the "batching" stage and we increase our luck to have everything in the same block.

Manually testing the MR

$ tezt -f evm_rollup.ml --title "Alpha: Check that the kernel can handle too many txs for a single run" --loop -j 7

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