Add support for Dependabot alert endpoints

[francois2metz]

This add support for reading dependabot alerts: https://docs.github.com/en/rest/dependabot/alerts

I added the First and Last parameters to the ListCursorOptions. It seems to replace the PerPage parameter.

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[gmlewis]

Thank you, @francois2metz !

[gmlewis]

@francois2metz - just FYI - no need to force-push to your PRs in this repo because we always squash-merge at the end anyway, and it is easier for reviewers if you don't. See CONTRIBUTING.md for more details. Also, please make sure to run gofmt on the modified files and go generate ./... before pushing. Thanks.

[gmlewis]

Thank you, @francois2metz !
LGTM.

Awaiting second LGTM+Approval from any other contributor to this repo before merging.

[gmlewis]

@francois2metz - please make sure to run gofmt and go generate ./... and push the results to make the tests pass.

[codecov]

Codecov Report

Merging #2554 (83796ef) into master (66a053a) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2554      +/-   ##
==========================================
+ Coverage   97.97%   97.98%   +0.01%     
==========================================
  Files         123      125       +2     
  Lines       10813    10873      +60     
==========================================
+ Hits        10594    10654      +60     
  Misses        150      150              
  Partials       69       69              

Impacted Files	Coverage Δ
github/github.go	97.37% <ø> (ø)
github/dependabot_alerts.go	100.00% <100.00%> (ø)
github/orgs_security_managers.go	100.00% <0.00%> (ø)

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[gmlewis]

Thank you, @valbeat !
Merging.