Update codeowners and relevant workflows

[Polber]

This will help auto-assign reviewers to core templates infra, with reviews for individual templates being left to author to decide.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.09%. Comparing base (babf762) to head (b34bd74).
Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1715      +/-   ##
============================================
+ Coverage     45.04%   45.09%   +0.05%     
- Complexity     3601     3934     +333     
============================================
  Files           835      835              
  Lines         49684    49693       +9     
  Branches       5229     5230       +1     
============================================
+ Hits          22378    22408      +30     
+ Misses        25639    25620      -19     
+ Partials       1667     1665       -2     

Components	Coverage Δ
spanner-templates	66.26% <50.00%> (+0.13%)	⬆️
spanner-import-export	64.22% <50.00%> (+0.40%)	⬆️
spanner-live-forward-migration	76.18% <ø> (ø)
spanner-live-reverse-replication	76.54% <ø> (ø)
spanner-bulk-migration	85.55% <ø> (ø)

see 5 files with indirect coverage changes

[damccorm]

Thanks! This overall LGTM, just had one comment

[damccorm]

Thanks!