feat(core): Add addEventProcessor method

[mydea]

And deprecate addGlobalEventProcessor() and getGlobalEventProcessors().

In v8, all event processors will be on the client only, streamlining this a bit and preventing global "pollution".

Closes #9082

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	65.87 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	59.47 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.12 KB (0%)
@sentry/browser - Webpack (gzipped)	21.38 KB (-0.01% 🔽)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	62.88 KB (+0.26% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	29.62 KB (+0.33% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	21.69 KB (+0.45% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	197.88 KB (+0.22% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	89.73 KB (+0.47% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	64.71 KB (+0.66% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	32.29 KB (+0.22% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	66.24 KB (-0.02% 🔽)
@sentry/react - Webpack (gzipped)	21.42 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	82.96 KB (-0.01% 🔽)
@sentry/nextjs Client - Webpack (gzipped)	48.21 KB (0%)
@sentry-internal/feedback - Webpack (gzipped)	16.17 KB (-0.13% 🔽)
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	74.52 KB (added)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	29.72 KB (added)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	71.13 KB (added)

[AbhiPrasad]

We need migration docs in MIGRATION.md

[mydea]

Good point!

Small note, with the upcoming hub & scope changes this also means that we'll update this to put the event processors on the isolation scope instead of the client, but that doesn't affect this PR (as it is a future change, that we can do under the hood in v8)