[Feature request] Layer to support delegating requests to arbitrary executor

[jlizen]

(Glad to contribute this, cutting an issue to gauge interest first)

Today we have the buffer layer which allows delegating requests to a background worker, but in serial.

I recently cut a change to reqwest that includes example code for a similar sort of layer, but this time processed in parallel by a different executor. It was fairly verbose boilerplate.

Would we want something similar in the tower's built in layers, under a feature flag? I could make it somewhat less opinionated by accepting an executor trait implementation. Or is that too niche to include in the main package?

Open to contributing something along these lines, but seeking feedback about what's useful (if anything :) )

[jlizen]

cc @seanmonstar as you have context on the above code