Expose when component model async tasks finish, not just return

Today Func::call_concurrent will resolve when the guest invokes task.return, but the guest can continue running. This concept, supported by the component model, should be modeled in Wasmtime's embedder API as well. For example the return value of call_concurrent could always return a WasmTask argument (or something like that) which can then be used to await with an &Accessor just as a function could be called concurrently. This would enable the embedder to, optionally, explicitly wait on guests that would like to continue running. Additionally the embedder could learn about traps during this time frame, but no other results would come out of the guest.

This makes a lot of sense! As an embedder I would expect to be able to acquire a construct similar to https://docs.rs/tokio/latest/tokio/task/struct.JoinSet.html