Allow strategy to be a modular compontent

[WardLT]

Is your feature request related to a problem? Please describe.
The strategies for how Parsl determines whether more or fewer nodes are required are defined in a single class, which makes experimenting with them difficult.

Describe the solution you'd like
Define an interface for the Strategy, allow the Config objects strategy field to receive a Strategy object in addition to stings defining the base types.

Describe alternatives you've considered
IDK, I guess I could use mock.pactch

Additional context
None

[benclifford]

As another alternative that doesn't involve mock: you can also, I think, use strategy='none' and implement your own separate strategy thread that calls into Parsl that isn't managed by the DFK.

I'd like to see strategy move towards the executors, as well as be more modular: it's not the business of the DFK to be understanding that a block provider executor like HTEX has blocks that can be scaled but the globus compute executor or the thread pool executor do not.

That might look like:

i) move strategy thread launch into block provider executor, with the outer loops rearranged/pruned so as to only deal with the current executor

ii) make this configurable per executor

[WardLT]

I too like the idea of moving things to the executor level, since it seems the strategies we have treat executors independently anyway