fix(go): support processing footers in gapics

[codyoss]

Because we need to filter the commits found in the footer of a
commit we needed a hook to filter commits for the changelog. This
can't be done in the releaser as it only has the concept of raw
commits.Added the idea of supplying an optional commit filter to
ConventionalCommits.

[codecov]

Codecov Report

Merging #711 (efa428a) into master (0258bd6) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #711      +/-   ##
==========================================
+ Coverage   86.96%   87.02%   +0.05%     
==========================================
  Files          55       55              
  Lines        6673     6696      +23     
  Branches      594      627      +33     
==========================================
+ Hits         5803     5827      +24     
+ Misses        869      868       -1     
  Partials        1        1              

Impacted Files	Coverage Δ
src/conventional-commits.ts	99.20% <100.00%> (+0.46%)	⬆️
src/releasers/go-yoshi.ts	94.59% <100.00%> (+0.25%)	⬆️

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[bcoe]

I like the approach you've come up with 👍 one thought, perhaps we could just default the filter to: () => true, then we don't need to do the optional call logic.