Allow returning executor handle from builder + calling execute with provided handle

[jlizen]

Libraries might want to allow callers to pass in config to load different executor strategies for different use cases.

This means that they need to allow selectively calling strategies per execute, rather than just using a globally configured strategy.

To accomplish this we need:

• the ability to return a strategy handle in the executor builder instead of installing it
• the ability to call execute with a handle (execute_with_strategy(), etc)

Also, we want the ability to point to the same strategy that is loaded globally via a handle. Since you don't want to spin up dupe threadpools maybe. I imagine this means that we should add the ability to directly install a strategy. So you first build it, get the handle, and then both pass it to the library as well as to an install function. Might take tweaking the bounds a bit.