feat(types): Add SerializedEvent interface (pre v8)

[Lms24]

This PR adds a SerializedEvent interface which more accurately represents what an event envelope item contains (i.e. the JSON structure of a serialized Event object). Specifically, the spans field now correctly only contains the attributes the POJO span will continue to hold, i.e. without the deprecations and methods declared in the Span interface.

In v8, we should change the EventItem type for the event envelope but doing so now is a breaking change I believe (And arguably, not too important to do now anyway).

However, we can already use this type in our browser integration tests to remove a lot of the unnecessary deprecation ignores.

ref #10222 (rest of this has to happen in v8)

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	77.37 KB (0%)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	68.67 KB (0%)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	72.54 KB (+0.08% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	62.31 KB (0%)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	32.67 KB (0%)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.13 KB (0%)
@sentry/browser - Webpack (gzipped)	22.48 KB (0%)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	75 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	66.63 KB (0%)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	32.48 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped)	24.19 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	209.91 KB (0%)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	97.89 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	72.34 KB (0%)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	35.49 KB (0%)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	69.06 KB (0%)
@sentry/react - Webpack (gzipped)	22.52 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	85.68 KB (0%)
@sentry/nextjs Client - Webpack (gzipped)	49.81 KB (0%)
@sentry-internal/feedback - Webpack (gzipped)	17.12 KB (+0.02% 🔺)