feat: get annotated tag message

[caarlos0]

this adds a new template field, .Subject, which will contain the annotated tag message if tag is annotated, or the message of the commit it points to.

This can be used on release.name_template, for example 🤘

closes #2726

[codecov]

Codecov Report

Merging #2730 (8bc3b80) into main (f9687b4) will decrease coverage by 0.01%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##             main    #2730      +/-   ##
==========================================
- Coverage   84.90%   84.88%   -0.02%     
==========================================
  Files         105      105              
  Lines        7901     7914      +13     
==========================================
+ Hits         6708     6718      +10     
- Misses        978      980       +2     
- Partials      215      216       +1     

Impacted Files	Coverage Δ
pkg/context/context.go	100.00% <ø> (ø)
internal/pipe/git/git.go	85.54% <57.14%> (-1.20%)	⬇️
internal/testlib/git.go	100.00% <100.00%> (ø)
internal/tmpl/tmpl.go	97.60% <100.00%> (+0.01%)	⬆️

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