Proto Alpha: Deprecate `consumed_gas` field in encodings.

Context

Partially resolve #2246 (closed) by deprecating consumed_gas field in protocol encodings in favour of consumed_milligas, which gives a more precise readout for the same value. Add consumed_milligas to the encodings where it is missing.

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