Refactor Variable CRUD methods in client

[aaazzam]

Related to #16472, this PR continues the optimization pattern established for Artifacts, though with a smaller impact since Variables is one of our lighter primitives. The memory footprint reduction is negligible in this case, but the PR:

• Refactors the client's Variable CRUD methods into a standalone module
• Uses ForwardRefs for heavy imports, moving them into the call itself

While the memory impact is minimal for Variables specifically, applying this pattern consistently across all primitives will help us reach our optimization targets in #16472. The refactoring maintains full feature parity while improving the module's organization and type safety.

fun fact: our previous async client didn't have a create method, while our sync one didn't have a read_variables method

[codspeed-hq]

CodSpeed Performance Report

Merging #16564 will not alter performance

_{Comparing variables-client (e5e4944) with main (277731d)}

Summary

✅ 3 untouched benchmarks

[desertaxle]

One comment on using request from the base clients for type safety on the routes, but otherwise LGTM! I like how PrefectClient shrinks a little bit with each of these PRs.

[desertaxle]

Do you want to use self.request for type safety here and in the other methods?

[aaazzam]

woof, yep!