feat: Add `tunnel` support to multiplexed transport

[timfish]

This PR:

• Adds a tunnel option to the internal transport options so it can be passed from client options
• Uses the tunnel option when creating the URLs in the multiplexed transport
• Replaces dsn in the envelope header with the correct dsn before sending
• Changes to using Map<string, Transport> to store the cached transports

Requires tests

[github-actions[bot]]

size-limit report 📦

Path	Size
@sentry/browser	21.65 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing)	32.69 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay)	68.03 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	61.43 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay with Canvas)	72.07 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback)	84.28 KB (+0.01% 🔺)
@sentry/browser (incl. Feedback)	37.79 KB (+0.02% 🔺)
@sentry/browser (incl. sendFeedback)	26.44 KB (+0.03% 🔺)
@sentry/browser (incl. FeedbackAsync)	30.94 KB (+0.02% 🔺)
@sentry/react	24.33 KB (+0.03% 🔺)
@sentry/react (incl. Tracing)	35.65 KB (+0.03% 🔺)
@sentry/vue	25.48 KB (+0.03% 🔺)
@sentry/vue (incl. Tracing)	34.48 KB (+0.02% 🔺)
@sentry/svelte	21.77 KB (+0.05% 🔺)
CDN Bundle	23.96 KB (+0.03% 🔺)
CDN Bundle (incl. Tracing)	33.98 KB (+0.02% 🔺)
CDN Bundle (incl. Tracing, Replay)	67.68 KB (+0.01% 🔺)
CDN Bundle (incl. Tracing, Replay, Feedback)	83.55 KB (+0.01% 🔺)
CDN Bundle - uncompressed	70.6 KB (+0.03% 🔺)
CDN Bundle (incl. Tracing) - uncompressed	100.96 KB (+0.03% 🔺)
CDN Bundle (incl. Tracing, Replay) - uncompressed	210.57 KB (+0.02% 🔺)
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	256.96 KB (+0.01% 🔺)
@sentry/nextjs (client)	34.87 KB (+0.02% 🔺)
@sentry/sveltekit (client)	33.25 KB (+0.03% 🔺)
@sentry/node	138.48 KB (+0.01% 🔺)