Pollable variant of RPC functions

[vigoo]

Part of golemcloud/golem#175

Changes the generated stubs in the following way:

• functions that return () already had two variants: blocking_xyz used invoke-and-await under the hood while xyz just used invoke to enqueue the invocation and return immeditaly.
• with this change, functions not returning () also have two variants: blocking_xyz is what was previously xyz - uses invoke-and-await under the hood and blocks until it gets aresponse.
• The xyz variant now returns a resource which have a subscribe and a get method - subscribe returns a pollable which can be used to wait until we get a response - get returns None if there is no response yet.

I plan to add helpers to the language-specific Golem wrappers (like golem-rust and golem-ts) to help processing these pollable responses in a more language idiomatic way.