Feature request: expose context contents to workers in other threads

[jsbueno]

Currently, contextvars -either stdlib's, or the objects in this project, offer a nice way to have each async task able to share contextual information (for example, information about an HTTP request) -

However, when doing asyncio programming a common pattern is to have some otherwise synchronous call to be executed in a thread-pool worker (for example, through an asyncio.loop.run_in_executor call).

The synch function called in this way is essentialy part of the task calling .run_in_executor - but there is no straightforward way to make the context variables in the main (async-) thread visible in the worker in the other threads. (any other thread starts with an empty context by default).

This is a natural fill-in for extra-context!