Adds more detailed telemetry fields

[rossgrambo]

Why this PR?

We want reader of telemetry to have more insight into the state of the feature flag.

• Adds additional telemetry fields.
• Added the same BytesExtension we have on the provider for Base64Url.
• Adds and adjusts unit tests

Visible Changes

• FeatureEvaluation events will have new fields:
  • VariantAssignmentPercentage - The percent of users we expect to be assigned the given variant for the given reason
  • DefaultWhenEnabled - The DefaultWhenEnabled field
  • AllocationId - An id that uniquely reflects the state of allocation. Two events with the same AllocationId can expect a given TargetingContext will always give the same result. **Note- This is instead computed in App Configuration Provider

[rossgrambo]

@zhiyuanliang-ms if you have time could you review this?