[Feature Request] Add feature flag support

[kinbiko]

What problem are you trying to solve?

The error reporting API has now been updated to support feature flags, and there's no reason not to allow this feature via this package.

Announcement

Describe how you wish this package would help you solve the problem

Add a WithFeatureFlag(ctx context.Context, featureFlag, variant string) context.Context function to the notifier, and pass this information along in error reports.

• WithFeatureFlag function in context.go
• Update/Add unit tests in context_test.go
  • In particular, check intended behaviour (serialization for both error reports and Serialize/Deserialize)
• Update integration test to include this feature.
• Update examples
• Add/update godocs
• Update "Differences with the official notifier"
• Release a new minor version

There does not seem to be a need to add this in sessions.