feat(core): Allow multiplexed transport to send to multiple releases

[timfish]

The multiplexed transport can already route events to different or multiple DSNs but we also need to be able to route to specific releases too.

In a page with micro-frontends, it's possible (and probably even quite common) to be using the same dependency multiple times but different versions (ie. different releases). Depending on where an error occurs we might want to send an event to [email protected] or [email protected] at the same DSN.

This PR:

• Adds a private makeOverrideReleaseTransport which can used to wrap a transport and override the release on all events
• Modifies makeMultiplexedTransport so it now creates a transport for each unique dsn/release pair
  • And uses makeOverrideReleaseTransport whenever a release is returned from the callback