Kernel/EVM: encapsulate TxCommon gas limit

Context

See !11512 (merged) for the why. Required for !11537 (merged).

The gas_limit is becoming a complex entity, as it is used both for execution gas fee, and additional fees. In order to deal with these two contexts correctly, we enforce that all uses for the purposes of computation, go through execution_gas_price.

Manually testing the MR

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