Create @ConjureGenerated Annotation

[kunalrkak]

Before this PR

Existing @Generated annotations have a retention policy of SOURCE and are discarded after compilation.
This makes it hard to later tell if an object is conjure generated or not, forcing us to try to pattern match based
on known characteristics, i.e. palantir/gradle-baseline#3344. Using an annotation with a CLASS retention
policy would allow us to just look for the presence of that annotation on a class instead.

After this PR

==COMMIT_MSG==
Replace @Generated annotations with @ConjureGenerated in generated code.
==COMMIT_MSG==

Possible downsides?

[changelog-app]

Generate changelog in changelog/@unreleased

Type (Select exactly one)

• Feature (Adding new functionality)
• Improvement (Improving existing functionality)
• Fix (Fixing an issue with existing functionality)
• Break (Creating a new major version by breaking public APIs)
• Deprecation (Removing functionality in a non-breaking way)
• Migration (Automatically moving data/functionality to a new system)

Description

Replace @Generated annotations with @ConjureGenerated in generated code.

Check the box to generate changelog(s)

• Generate changelog entry

[kunalrkak]

I left this as a String to mirror the existing usage of @Generated but given we're exposing this more broadly, perhaps we should switch to an enum of supported types?

[kunalrkak]

It looks like ErrorProne is wired up (with this config) to skip over code tagged with the @Ganerated annotation, which is why it's now complaining about some of the integration input files.