Introduce local-only action utils

[DaveCTurner]

Introduces some never-called localOnly() placeholders to clearly mark
the dead code in actions that never run on remote nodes.

Relates #100111

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[ldematte]

LGTM, but I have some questions.

I suspect this covers only a couple of example usages, and we would need to follow up with some other PRs to convert other existing usages, correct?
These replacements look safe as behaviour is unchanged, and probably other direct usages of TransportAction will too (as their implementation is already empty/throwing). But what about HandledTransportAction? Is there a way we can see which ones are local only, and introduce this pattern/a similar pattern?

[DaveCTurner]

Is there a way we can see which ones are local only, and introduce this pattern/a similar pattern?

Not easily, the bulk of the work for #100111 is going to be looking at all the other actions one-by-one and separating them out into local-only and remote-cluster actions.

[ldematte]

the bulk of the work for #100111 is going to be looking at all the other actions one-by-one and separating them out into local-only and remote-cluster actions.

I had a suspect that was the case :/
Do you think that "manual inspection" would be enough (e.g. can we isolate which actions are not used with RemoteClusterAwareClient and infer they are local-only) or should we think about a way of validating this first, e.g. by "instrumenting" all actions with logs, run tests/deploy in Staging and collect info?

[DaveCTurner]

I think I'd approach it by separating the remote-cluster actions from the local-cluster ones first, using the type system to catch any mistakes. However I would expect this all to be fairly well-covered by tests (particularly REST tests) so as long as the tests pass we're probably ok.