feat(flags): capture feature flag evaluations on spans

[aliu39]

Based off https://github.com/getsentry/sentry-python/pull/4280/files
Reopened from #16475

This PR updates our 5 browser FF integrations to capture flag evaluations on spans. This implementation avoids changes to the Span class, saving bundle size.

• on eval, flags are buffered per-span in a global WeakMap.
• the .on('spanEnd') hook is used to freeze the flags in the span attributes when the span finishes.
• staying consistent with the python PR:
  • we only capture values for the first 10 unique flags per span. These are in order of last evaluation, but subsequent flags are dropped.
  • attribute keys have the format flag.evaluation.{flagKey}

From @AbhiPrasad :

A method on the span break our compat with OTEL spans, which we use for duck typing in some scenarios.

From @cmanallen :

For spans the oldest flag evaluations are the most important because they're likely to have the largest performance impact. We drop late arrivals due to request size concerns.

[aliu39]

Useful refs:

• https://develop.sentry.dev/sdk/platform-specifics/javascript-sdks/browser-tracing/
• https://develop.sentry.dev/sdk/data-model/event-payloads/span/