Use `tunnel` transport rather than top level `tunnel` option

[timfish]

Problem Statement

Currently to use the tunnel option, you pass it in the top level init options:

Sentry.init({
  dsn: '__DSN__',
  tunnel: '/sentry-tunnel',
})

But this means that the tunnel value gets passed around throughout the code-base because it is required for every different type of envelope creation.

Solution Brainstorm

Use a transport instead:

Sentry.init({
  dsn: '__DSN__',
  transport: Sentry.tunnel('/sentry-tunnel'),
});

The tunnel transport would wrap the default transport for that SDK and inject the DSN into the envelope headers and override the URL.

[AbhiPrasad]

I think as per discussions we're actually going to do an integration (with some new hooks) to make this work.

Going to close this so that #14248 becomes the sole source of truth.