refactor: execution methods into Executor class

[yaacovCR]

see: #3163 (comment)

depends on #3184

[yaacovCR]

Hi @IvanGoncharov

I see your point, that is functionally equivalent, and it does seem cleaner.

So....I think I have the wrapping execute and subscribe functions as slim as they can go, and I have combined the Executor and SubscriberExecutor classes into one.

This is ready again for review. Please feel free to edit/rename/reformat within this PR as necessary.

I am feeling very good about this! I think this will unlock a lot of potential for experimentation from which the ecosystem will benefit greatly. I'd love to see this within the next alpha.

[yaacovCR]

In particular, I don't love how I left the organization of folders/directories. The breakdown in structure between execution and subscription does not seem to make sense anymore, as all the essential logic anyway is within the Executor class in the executor.ts. One thought would be to move all the subscription files into the execution folder also -- but it is somewhat helpful having more folders to help keep the tests organized. Back on the other hand, the subscription tests are really just testing the Executor, which doesn't live in that folder. I am going to push one more commit that just moves everything to execution, but feel free to change this PR at will.

[yaacovCR]

merging this into branch on main repo, will split out into separate PRs as possible