This class is modeled after PocketEndpointRaw from FFTV but adapted to:
- Take more dependencies as args instead of statically
- Use :concept-fetch
- Be more generic to more gracefully handle additional APIs

PocketEndpoint is intended to wrap this class and convert the raw JSON
into Kotlin types useful to the application.

See PocketEndpointRaw from FFTV:
  https://github.com/mozilla-mobile/firefox-tv/blob/daf118b092a7083cd41fdd51c5448fa41e4a885d/app/src/main/java/org/mozilla/tv/firefox/pocket/PocketEndpoint.kt#L75

This is intended to be passed in as a dependency to the endpoint classes
for them to know which urls to access.

This was adapted from Firefox for Fire TV's
BuildConfigDerivables.globalPocketVideoEndpoint:
https://github.com/mozilla-mobile/firefox-tv/blob/daf118b092a7083cd41fdd51c5448fa41e4a885d/app/src/main/java/org/mozilla/tv/firefox/utils/BuildConfigDerivables.kt#L30

This exposes two new testing only functions in `Glean`:
`testGetExperimentData` and `testIsExperimentActive`. This
additionally fixes Glean test code so that metrics clearing
only happens once.

This moves the validation that was happening in the `*MetricType` classes to their associated `*StorageEngine` classes.
Removing TODO's and removing storage engine references from metrics
Move tests involved with data validation from metrics to storage engines

This was modeled after the commit that added :service-glean:
92a936c7

Add description of background state, as well as add subsections and make all the specific ping docs look similar. This included adding a summary description, scheduling, and content information sections. Added links to new background definition where appropriate.  Finally, updated the events ping documentation to reflect recent changes to the schema with the removal of `objectId` and `value` fields.
Additional changes to docs per Bug 1525578