feat(extend-app-start): App start extension API

[buenaflor]

PR Stack (Extend App Start)

• feat(extend-app-start): App start extension API #5604
• feat(extend-app-start): [1/4] Add IAppStartExtender bridge #5605
• feat(extend-app-start): [2/4] Extract AppStartExtension component #5606
• feat(extend-app-start): [3/4] Eagerly create the extended app start transaction #5608
• feat(extend-app-start): [4/4] Add public Sentry app start extension API and update example app #5628

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Tested scenarios:

1 · Non-headless — extended after first frame (finish called ~5s in) → OK, app_start_cold = 6511 ms
https://sentry-sdks.sentry.io/explore/traces/trace/cb4ce5b65c42445da7d87d80fd49896a/

2 · Non-headless — finish never called → DEADLINE_EXCEEDED, measurement suppressed
https://sentry-sdks.sentry.io/explore/traces/trace/87bb5f4916f04837b5b4b6a31b021d0a/

3 · Headless — extended (finish called) → OK, app_start_cold = 6342 ms
https://sentry-sdks.sentry.io/explore/traces/trace/0563146982d44e05ac67000cc0089f6e/

4 · Headless — finish never called → DEADLINE_EXCEEDED, measurement suppressed
https://sentry-sdks.sentry.io/explore/traces/trace/6aa847bb646c4810b2991ad6b1c2687d/

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Collection PR for the Extend App Start stack. Squash-merge this into main once all stack PRs are merged.

Implements the app start extension API — Sentry.extendAppStart() / Sentry.finishAppStart() / Sentry.getExtendedAppStartSpan() — which lets apps extend the app start measurement to cover extra launch-time work.

Closes #5553 #3581

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