Add SecretScanningAlertEvent Webhook

[IainSteers]

Fixes#2277

[codecov]

Codecov Report

Merging #2282 (1c8a0ca) into master (a842efc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@ Coverage Diff @@## master #2282 +/- ##
=======================================
Coverage 97.83% 97.83% =======================================
Files 115 115 Lines 10360 10362 +2 =======================================
+ Hits 10136 10138 +2 
Misses 156 156 Partials 68 68 

Impacted Files	Coverage Δ
github/event_types.go	100.00% <ø> (ø)
github/messages.go	100.00% <ø> (ø)
github/event.go	100.00% <100.00%> (ø)

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[IainSteers]

It looks like #2281 introduces the full SecretScanningAlert struct used in the API.

I propose waiting until that merges and then I'll refactor to use it in the webhook payload.

[IainSteers]

@gmlewis this is good to go for review. I rebased it on Friday to use the SecretScanningAlert

[gmlewis]

Thank you, @IainSteers !
Just a couple minor tweaks, please, for the auto-generated godocs.
Also, please refrain from using force-push in this repo... we squash all commits anyway, and it is easier for the reviewers to see what changed. For more info, please see our CONTRIBUTING.md doc.

[IainSteers]

Apologies for the force-push, did it whilst rebasing without realising. I've made the corrections in punctuation.

[gmlewis]

Thank you, @IainSteers!
LGTM.

Awaiting second LGTM/Approval from any other contributor of this repo before merging.

[raynigon]

LGTM

[gmlewis]

Thank you, @raynigon !
Merging.