Proto+Plugin/Michelson: enrich execution traces with "just consumed gas"

Context

This MR changes the gas information displayed in execution traces. Instead of reporting at each step the amount of remaining gas, it reports the amount of gas consumed during the execution of the last step. The goal is twofold:

• simplify studying gas consumption in execution traces: identifying cheap and expensive steps gets easier,
• make regression diffs less verbose: only the step at which a gas cost changes should be reported (instead of all remaining steps).

This was developed while working on #3971 (closed) but is more broadly useful.

Depends on !6546 (merged).

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