Support different coroutine models in workflows and activities

Describe the solution you'd like

Need to several things:

• May need to slightly abstract workflow concurrency impl to support (but may not)
• Update Kotlin project to add support for Kotlin coroutines in workflows and activities as Kotlin suspend fun calls (may need to be opt-in in some way). There is an internal POC implementation from @mfateev we can leverage here.
• Create a new Java 21+ only "temporal-virtual-threads" project that uses virtual threads for workflow concurrency and runs activities in virtual threads (may need to be opt-in in some way)

This is kinda a big project, so some discussion should happen first. The above is just speculation about what to do.

Start by reviewing the workflow concurrency implementation and the Kotlin project, then examine the internal coroutine proof of concept referenced in the issue. The proposed work also includes a new Java 21+ temporal-virtual-threads project. Done would require an agreed design and support for the requested coroutine and virtual-thread models, but no specific files or tests are named.