Add email callback on train complete

[loucaspapalazarou]

What does this PR do?

This PR adds a new callback that allows a user to notify a list of emails when the training is done. It currently only supports the Google SMTP and is recommended to use an app password. Adding more providers is trivial. If the feedback is positive, I will go ahead and add the docs update as well as proper tests.

Resolves #20133

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📚 Documentation preview 📚: https://pytorch-lightning--20162.org.readthedocs.build/en/20162/

[codecov]

Codecov Report

Attention: Patch coverage is 37.73585% with 33 lines in your changes missing coverage. Please review.

Project coverage is 81%. Comparing base (474bdd0) to head (45139a1).
Report is 144 commits behind head on master.

❌ Your patch check has failed because the patch coverage (38%) is below the target coverage (50%). You can increase the patch coverage or adjust the target coverage.

❗ There is a different number of reports uploaded between BASE (474bdd0) and HEAD (45139a1). Click for more details.

HEAD has 87 uploads less than BASE

Flag	BASE (474bdd0)	HEAD (45139a1)
cpu	41	21
python3.10	12	6
lightning_fabric	7	0
pytest	25	2
lightning	31	16
python3.9	11	6
gpu	4	2
python3.11	12	6
python3.12	6	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20162     +/-   ##
=========================================
- Coverage      89%      81%     -8%     
=========================================
  Files         267      265      -2     
  Lines       23064    23062      -2     
=========================================
- Hits        20573    18626   -1947     
- Misses       2491     4436   +1945     

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