feat(flags): add Statsig browser integration

[aliu39]

Ref https://develop.sentry.dev/sdk/expected-features/#feature-flags. Adds an integration for tracking Statsig js-client flag evaluations, specifically the checkGate method which is used for boolean release flags.

Statsig references:

• https://docs.statsig.com/client/javascript-sdk
• https://docs.statsig.com/client/javascript-sdk#client-event-emitter
• Event emitter types definitions

Our current FF integrations only support boolean flag values so "dynamic config", "experiments", and "layers" will not be tracked for now.

Closes https://github.com/getsentry/team-replay/issues/539

[aliu39]

Excluded props:
// readonly ruleID: string;
// readonly details: EvaluationDetails;
// readonly __evaluation: GateEvaluation | null;

[aliu39]

Full list of events at https://github.com/statsig-io/js-client-monorepo/blob/5800b1c771c7cec491c82e3a714475d678ca284b/packages/client-core/src/StatsigClientEventEmitter.ts#L24-63

[codecov]

❌ 2 Tests Failed:

Tests completed	Failed	Passed	Skipped
635	2	633	302

View the top 2 failed test(s) by shortest run time

tracing/browserTracingIntegration/long-tasks-before-navigation/test.ts doesn't capture long task spans starting before a navigation in the navigation transaction

Stack Traces | 30s run time

test.ts:7:11 doesn't capture long task spans starting before a navigation in the navigation transaction

tracing/dsc-txn-name-update/test.ts updates the DSC when the txn name is updated and high-quality

Stack Traces | 30s run time

test.ts:12:11 updates the DSC when the txn name is updated and high-quality

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[billyvg]

Does our unleash SDK have users? best practice would be to add featureFlagClient arg to unleash integration and keep unleashClient as optional but deprecated - then in a breaking release we can remove. But actually if it's only published in v9 branch it's probably fine to break it now (not in this PR).

[aliu39]

Does our unleash SDK have users? best practice would be to add featureFlagClient arg to unleash integration and keep unleashClient as optional but deprecated - then in a breaking release we can remove. But actually if it's only published in v9 branch it's probably fine to break it now (not in this PR).

It's released in v8 as well, I'm not sure how many users