feat: Add feature flag support

[jespersandnielsen]

Add Sentry.add_feature_flag and Scope#add_feature_flag to record boolean flag evaluations. Flags are kept in a 100-entry LRU buffer on the scope, attached to error events as contexts.flags.values (not transactions), and recorded on the active span as flag.evaluation.* data (max 10 per span).

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Description

Describe your changes:

• resolves: Support for tracking feature flags #2559