feat(v7): Add tunnel support to multiplexed transport

[timfish]

Backport of #11806

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	80.77 KB (added)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	71.75 KB (added)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	76 KB (added)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	65.37 KB (added)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	35.63 KB (added)
@sentry/browser (incl. browserTracingIntegration) - Webpack (gzipped)	35.51 KB (added)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.57 KB (added)
@sentry/browser (incl. sendFeedback) - Webpack (gzipped)	31.58 KB (added)
@sentry/browser - Webpack (gzipped)	22.78 KB (added)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	78.98 KB (added)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	70.35 KB (added)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	36.03 KB (added)
@sentry/browser - ES6 CDN Bundle (gzipped)	25.28 KB (added)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	221.43 KB (added)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	109.03 KB (added)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	75.81 KB (added)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	39.3 KB (added)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	72.25 KB (added)
@sentry/react - Webpack (gzipped)	22.81 KB (added)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	90.01 KB (added)
@sentry/nextjs Client - Webpack (gzipped)	54.14 KB (added)
@sentry-internal/feedback - Webpack (gzipped)	17.32 KB (added)