Add async support to the component model C API

[chaynabors]

Closes #12955

Adds async equivalents of the component model C API, following the existing core module async patterns in async.h.

New functions:

• wasm_component_model_async
• wasm_component_model_async_builtins
• wasm_component_model_async_stackful
• wasmtime_component_func_call_async
• wasmtime_component_linker_instantiate_async
• wasmtime_component_linker_instance_add_func_async
• wasmtime_component_linker_add_wasip2_async
• wasmtime_component_linker_add_wasi_http_async

A few notes on the design:

• No store-level value storage reuse (unlike core module's wasm_val_storage). The sync component model C API doesn't have it either, so this could be a follow-up for both sync and async.
• C++ header only wraps the WASI/HTTP linker functions since the future-returning functions would need some fort of CallFuture wrapper that doesn't exist for core modules either. Should I add one?
• Feature gated behind component-model-async, enabled by default in release builds. Should this be disabled by default?
• No C API tests for async exist in the codebase today, including for non-component modules. Happy to add some scaffolding if we want it, and have a preference for how it's done.
• I include async.h at the top with the rest of the includes. It's harmless since the file itself is entirely conditional in the interior but the import could be inlined further down. There wasn't any precedence and that gets messy so I didn't.

[alexcrichton]

Thanks for this! Just some minor stylistic comments below, but overall looks great.

There's currently no tests in this PR, and while I realize that we don't currently have tests for async in the C/C++ APIs, would you be up for adding some at least simple tests for calling functions/host functions/etc?

[alexcrichton]

Could this go within the above WASMTIME_FEATURE_COMPONENT_MODEL block to avoid needing to redo the extern "C" bits?

[alexcrichton]

Similar to above, can this block move upwards to avoid duplicating the extern "C"?