add support for generic webhooks

[goedelsoup]

Adds support for webhook notifications with a minimal authorization provider implementation.

[codecov-io]

Codecov Report

Merging #183 into master will decrease coverage by 0.2%. The diff coverage is 39.62%.

@@            Coverage Diff             @@
##           master     #183      +/-   ##
==========================================
- Coverage   55.48%   55.27%   -0.21%     
==========================================
  Files         132      134       +2     
  Lines        4351     4400      +49     
  Branches      107      112       +5     
==========================================
+ Hits         2414     2432      +18     
- Misses       1937     1968      +31

Impacted Files	Coverage Δ
core/src/main/scala/notifications/WebHook.scala	0% <0%> (ø)
...re/src/main/scala/notifications/Notification.scala	100% <100%> (ø)	:arrow_up:
core/src/main/scala/Config.scala	76.8% <100%> (+0.17%)	:arrow_up:
core/src/main/scala/notifications/Notify.scala	53.65% <33.33%> (-1.9%)	:arrow_down:
...c/main/scala/notifications/NotificationEvent.scala	4.76% <4.76%> (ø)
core/src/main/scala/yaml/ManifestV1Parser.scala	78.47% <93.33%> (+0.69%)	:arrow_up:

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[goedelsoup]

Realized this requires users to configure secrets into the manifest, which is unacceptable. I'll research having named hooks, which are set up in the nelson config, or allowing a configuration routine that fetches credentials securely from Vault.