Smart rollup node: Context garbage collection

Context

This MR enables garbage collection for the context of the rollup node. In doing so, it introduces 2 new node parameters, which can be set through command line arguments:

• GC retention period: how many L2 blocks the node always needs to keep. The default is 2x the refutation window on Mainnet and it can only be overridden with a larger value. After review, the number of blocks to keep in the default operating mode is based around the lcc. A retention period argument will be introduced in a later MR which implements history modes.
• GC frequency: number of blocks between GC runs. The default is 100.

Manually testing the MR

dune exec tezt/tests/main.exe -- rollup_node gc

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