Setup mirror to route Sentry traffic to

[sentrivana]

There are parts of Sentry that need to send envelopes to a different DSN to avoid cycles. Currently, this is done via a custom multiplexing transport. Instead, we want to send to one mirror which will determine which instance to send to.

This issue is about creating the ops PR to create the mirror. #6533 is then the Sentry codebase cleanup part.

[linear-code]

PY-2518