Expose worker forking as a host function

[vigoo]

We want to expose #1138 as a host function too - so it is possible to create a fork from a worker.
This is aligned with our goal of not introducing any external APIs without providing the same functionality as a host function too.

The steps to implement this:

• Define the new function in golem-wit, in the golem:api/[email protected] interface
• It should be a function taking the same three parameters as the gRPC API
• Implement it in worker executor base using the worker proxy (as the forked worker may not be living in the same executor that initiated the fork); see how the hot update host function is implemented for reference.