More client refactoring & add versioning-opt-in config flag

[Sushisource]

What was changed

• Shuffled a bunch of stuff around to make the worker client easier to use / enable this feature
• Added bool flag to WorkerConfig for opting in to versioning feature

Why?

Will be needed to test versioning from a real lang SDK.

Relates to https://github.com/temporalio/sdk-features/issues/84

[Sushisource]

Literally everything is all-internal (or super-low external impact) refactoring besides the last commit to add the flag: https://github.com/temporalio/sdk-core/pull/374/commits/4eb253443cf0ec4446e63f14c99ba328fa673deb

[Sushisource]

Wondering if Core should protect when asked to use buildId based versioning and server doesn't have that capability

Makes sense, added issue here https://github.com/temporalio/temporal/issues/3230