Allow formatters/linters/testers to operate on generated code

[sureshjoshi]

From Slack:
https://pantsbuild.slack.com/archives/C046T6T9U/p1778251384271129

In digging through the codebase, we allow generated code to take part in certain processes, case-by-case.

I wonder if we could just register a plugin more generally to let the user opt-in to how SourceRequests/Hydration takes place (ideally on a target-ish level, not globally).

[sureshjoshi]

There are a lot of other tickets dealing with manipulation of generated code (e.g. #17042).

I wonder if there is some value in elevating generated code status from "some hidden away thing" to "here's the thing"