[git] Add scafolding for commit message standardization

[ricab]

Add a commit message template and a python script to serve as a validation hook.

MULTI-2014, MULTI-2015, MULTI-2017

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.31%. Comparing base (5598edb) to head (fc0e0c5).

Additional details and impacted files

@@                   Coverage Diff                    @@
##           contribution-guidelines    #4129   +/-   ##
========================================================
  Coverage                    89.31%   89.31%           
========================================================
  Files                          259      259           
  Lines                        15684    15684           
========================================================
  Hits                         14008    14008           
  Misses                        1676     1676           

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[ricab]

OK, I think this is ready (provided the base merges).

Here is a workflow run with bad commits: https://github.com/canonical/multipass/actions/runs/16480729443/job/46594055132?pr=4272

@sharder996, would you recommend having this workflow in the branch protection, or as part of dynamic CI?