Start porting mapped task to SDK

[ashb]

This PR restructures the Mapped Operator and Mapped Task Group code to live in
the Task SDK at definition time.

The big thing this change does not do is make it possible to execute mapped
tasks via the Task Execution API server etc -- that is up next (#44360).

There were some un-avoidable changes to the scheduler/expansion part of mapped
tasks here. Of note:

BaseOperator.get_mapped_ti_count has moved from an instance method on
BaseOperator to be a class method. The reason for this was that with the move
of more and more of the "definition time" code into the TaskSDK BaseOperator
and AbstractOperator it is no longer possible to add DB-accessing code to a
base class and have it apply to the subclasses. (i.e.
airflow.models.abstractoperator.AbstractOperator is now not always in the
MRO for tasks. Eventually that class will be deleted, but not yet)

On a similar vein XComArg's get_task_map_length is also moved to a single
dispatch class method on the TaskMap model since now the definition time
objects live in the TaskSDK, and there is no realistic way to get a per-type
subclass with DB logic (i.e. it's very complex to end up with a
PlainDBXComArg, a MapDBXComArg, etc. that we can attach the method too)

For those who aren't aware, singledispatch (and singledispatchmethod) are a
part of the standard library when the type of the first argument is used to
determine which implementation to call. If you are familiar with C++ or Java
this is very similar to method overloading, the one caveat is that it only
examines the type of the first argument, not the full signature.

The long term goal here is to have a clean separation between "runtime/definition time" behaviour (i.e. creating mapped tasks, or running a mapped task) and expanding a mapped task (which is a scheduling-time operation only)

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[ashb]

Mypy is seriously unhappy. Oh well

[ashb]

Oh also singlediaptch and singledispathmethod don't play great with type hints in 3.9. Worked around that easily enough now though.

[kaxil]

unrelated: ValidationSource is a weird name but it existed before the PR anyway

[kaxil]

First pass, will do a more detailed look in an hour

[kaxil]

Few comments but the code looks good, minor adjustments needed to get tests passing