Consider adding an opt-in mechanism to request that a host keeps a task alive

This is spun out of discussion from https://github.com/WebAssembly/component-model/issues/608. The general idea here is that similar to waitUntil on the web platform tasks could have an opt-in mechanism to provide a future<_> to the host and say "please consider me alive until this is resolved". This could affect behavior on the host of when an instance or store is shut down. For example using this API could keep wasi:cli/run alive longer or maybe it could keep a wasi:http/service still running.

This is related to https://github.com/WebAssembly/component-model/issues/611 insofar as it has to do with component teardown/shutdown from a host's perspective, and this is ideally a bit more cooperative than "host kills guest" unconditionally.

Start by reading the discussion in issue 608 and the related teardown discussion in issue 611. Compare the proposed waitUntil-like behavior with the wasi:cli/run and wasi:http/service examples. Done means reaching agreement on an opt-in host-lifecycle mechanism and documenting its effect on instance or store shutdown.