Shell & plugins: bound the mempool by op count and total byte size

Context

Fixes #5196 (closed)

The main semantic change in this MR consists in new bounding constraints on the valid operations kept in the mempool:

• Before, we ensured that the number of valid manager operations was at most max_prechecked_manager_operations = 5000, with no constraints on the other kinds of operations.
• Now, we ensure that the number of valid operations of any kind is at most max_operations = 10_000, and also the sum of the sizes in bytes of all valid operations is at most max_total_bytes = 10_000_000.

The values of max_operations and max_total_bytes can be retrieved with GET /chains/<chain>/mempool/filter and configured with POST /chains/<chain>/mempool/filter (just as we could do with max_prechecked_manager_operations before).

This MR also introduces significant engineering changes, in that the bounding of the mempool is now handled inside of lib_shell (in new file prevalidator_bounding.ml) instead of in the plugin. As a result, significant parts of the mempool plugin have been removed; notably, the plugin no longer maintains an internal state of valid operations.

Follow-up issue: #5541 (closed)

MR that depends on this one: !8573 (merged)

Depends on: !7702 (merged) !8501 (merged)

Manually testing the MR

Green CI, in particular the new/updated tests:

dune exec tezt/tests/main.exe -- mempool bounding
dune exec tezt/tests/main.exe -- mempool filter
dune exec tezt/tests/main.exe -- mempool rpc regression
dune exec src/lib_shell/test/main.exe -- -f test_prevalidation.ml

You may also, in sandbox, use POST /chains/<chain>/mempool/filter to change max_operations and max_total_bytes to low values, then try and inject too many / too large operations.

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).
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