feat(events): add PR merged and PR closed webhook events

[ecrupper]

closes go-vela/community#1030

adding a new webhook event action as a Vela capability.

PR merged payload has a merge commit that can be grabbed to use as the ref and commit.

PR closed I decided to use the base node for ref and commit to avoid any sort of nefarious activity.

[codecov]

Codecov Report

Attention: Patch coverage is 59.25926% with 55 lines in your changes missing coverage. Please review.

Project coverage is 56.63%. Comparing base (ff7c877) to head (0f112b6).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
api/webhook/post.go	0.00%	28 Missing ⚠️
api/build/compile_publish.go	0.00%	11 Missing ⚠️
scm/github/webhook.go	70.00%	4 Missing and 2 partials ⚠️
api/types/build.go	80.76%	2 Missing and 3 partials ⚠️
api/types/actions/pull.go	90.62%	2 Missing and 1 partial ⚠️
util/util.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1264      +/-   ##
==========================================
+ Coverage   56.60%   56.63%   +0.03%     
==========================================
  Files         629      629              
  Lines       35653    35763     +110     
==========================================
+ Hits        20180    20254      +74     
- Misses      14792    14824      +32     
- Partials      681      685       +4     

Files with missing lines	Coverage Δ
api/types/events.go	100.00% <100.00%> (ø)
database/testutils/api_resources.go	94.90% <100.00%> (+0.03%)	⬆️
util/util.go	25.30% <0.00%> (-0.63%)	⬇️
api/types/actions/pull.go	93.33% <90.62%> (-0.85%)	⬇️
api/types/build.go	96.87% <80.76%> (-0.70%)	⬇️
scm/github/webhook.go	90.01% <70.00%> (-0.97%)	⬇️
api/build/compile_publish.go	0.00% <0.00%> (ø)
api/webhook/post.go	0.00% <0.00%> (ø)

... and 3 files with indirect coverage changes