Change the return type of tune() in trainer.py to TypedDict

[donlapark]

What does this PR do?

Change the type of return of tune() in pytorch_lightning/trainer/trainer.py to TypedDict. (related to #13616 and #13445)
More precisely, a new type _TunerResult, as suggested by @carmocca in #13616, has been defined in pytorch_lightning/tuner/tuning.py and assigned to the output of tune().

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[carmocca]

Cool!

[otaj]

Some tests seem to be not happy with NotRequired. It seems, our typing_extensions might be too old, since we require >=4.0.0, but according to this, there was a fix for making NotRequired available within __all__ only in 4.1.0

[carmocca]

Then let's upgrade the minimum typing_extensions version