Add support for the secret scanning API

[crqrdotcom]

This PR adds support for the Secret Scanning API. Includes all endpoints and structures listed on the API documentation.

Closes#2278

[gmlewis]

Thank you, @crqra !

[codecov]

Codecov Report

Merging #2281 (000bc40) into master (e1c28bc) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@ Coverage Diff @@## master #2281 +/- ##
==========================================
+ Coverage 97.81% 97.83% +0.01% 
==========================================
Files 114 115 +1 Lines 10266 10357 +91 ==========================================
+ Hits 10042 10133 +91 
Misses 156 156 Partials 68 68 

Impacted Files	Coverage Δ
github/github.go	97.63% <100.00%> (+<0.01%)	⬆️
github/secret_scanning.go	100.00% <100.00%> (ø)
github/event_types.go	100.00% <0.00%> (ø)

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[crqrdotcom]

Thanks for the feedback @gmlewis! I've updated the PR with your suggestions, LMK if it looks good 😄

[gmlewis]

Thank you, @crqra !
LGTM.
Awaiting second LGTM/Approval before merging.

[gmlewis]

@Parker77 - do you have time to review this PR?

[Parker77]

LGTM.

[gmlewis]

Thank you, @Parker77 !
Merging.