Allow users to choose the exectuor for each async provider

[mxgrey]

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Description

We currently have an async_execution module that handles the logic to switch between bevy's AsyncComputeTaskPool and our built-in SingleThreadedExecution task executors based on whether the user has chosen to activate the single_threaded_async feature.

In principle we could have a mechanism to let users choose a specific async executor for each provider that they use. This could be advantageous if they generally want to use the AsyncComputeTaskPool but sometimes need to run a Future that does not implement Send (in which case it needs to be run in the single-threaded executor). It's also possible that a user might have their own custom executor that they want to use, perhaps for only some of their async providers.

The major challenges are:

• Define stable public traits that let users inject custom executors
• Define a reasonable API for specifying what executor each async provider should use