feat: generate release notes support

[caarlos0]

supports the new feature github announced today: generate release notes (link)

while there are no official docs yet, one of the product managers posted screenshots of it on twitter: https://twitter.com/MylesBorins/status/1445102641214861318

This implements both the new /releases/generate-notes endpoint and the new generate_release_notes release option.

[codecov]

Codecov Report

Merging #2114 (2b34cb0) into master (3ad22bc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2114   +/-   ##
=======================================
  Coverage   97.78%   97.78%           
=======================================
  Files         112      112           
  Lines        9956     9970   +14     
=======================================
+ Hits         9735     9749   +14     
  Misses        154      154           
  Partials       67       67           

Impacted Files	Coverage Δ
github/repos_releases.go	89.40% <100.00%> (+0.66%)	⬆️

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[gmlewis]

Thank you, @caarlos0 !
Just a few minor tweaks, please.

Also, to clarify, you can drop the omitempty and the pointer to a field if you know they are absolutely mandatory.

[caarlos0]

Thanks for the review @gmlewis! Just fixed the based on this tweet.

[gmlewis]

Thank you, @caarlos0 !
LGTM.

Awaiting second LGTM (from any other contributor to this repo) before merging.

[caarlos0]

can someone else review/merge this?

[gmlewis]

@Parker77 - do you have time to review this?

[Parker77]

LGTM.

[gmlewis]

LGTM.

Thank you, @Parker77!
Merging.