Executor Import/Load time optimization

[o-nikolas]

Overview

This PR aims to improve the time it takes to load/import the various executor modules we have in Airflow.

Motivation

The executors are imported in more places now that various compatibility checks are in core Airflow code (re: AIP-51). Also, decreasing import times is very important for the work of executors vending CLI commands (see #29055), since the CLI code in Airflow is particularly sensitive to slow imports (because all code is loaded fresh each time you run an individual Airflow CLI command).

The changes

This PR mostly includes changes to move some expensive imports that are only used for type checking under the TYPE_CHECKING flag so that they are not run at runtime. As well as moving a select few expensive imports closer to the code which uses them.
The most important changes are in the BaseExecutor module since all other executors load this module, and so benefits made here propagate outward.

Testing

I benchmarked these changes by writing a script to import the various executor modules in a fresh python runtime and timing how long that takes (you can test this yourself quickly from a bash shell by doing something like time python -c 'from airflow.executors.local_executor import LocalExecutor'). Then doing that in a loop for several samples (with some randomness in the order for fairness) both on main and on my development branch.

Results

Most executors saw a ~50% speed increase. Kubernetes, and to a lesser extent Celery, are still quite slow and will need more changes specifically targeted to those modules (in a future PR).
The combined executors (e.g. LocalKubernetesExecutor) saw less gains since they import two executors each, so they're paying double the cost (so they saw half the gains, 25%)

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[o-nikolas]

Fresh rebase from main and this is still green. Anyone have time to review/approve? (CC @uranusjr @potiuk @eladkal)

[uranusjr]

I still suspect some of the local imports are not necessary, but those are simple enough as mentioned and therefore introduce no downsides.