rfcs: sqlproxy connection migration

This proposal describes the connection migration mechanism within the SQL
Proxy that allows us to migrate SQL connections from one SQL pod to another
by building on top of the session migration work by the SQL Experience team.

This RFC is only scoped to _how_ to perform the migration from one SQL pod to
another within the SQL proxy. The details of _when_ to perform the migration
will be discussed in a follow-up RFC. Note that the main use-case here is for
CockroachDB Serverless, and we should optimize for that.

What this will eventually solve:
- rude disconnects during Serverless scale-down events
- load imbalance during steady state and Serverless scale-up events

From the user's perspective, the connection should remain usable without
interruption, and there will be better resource utilization across all
provisioned SQL pods.

Release justification: RFC/docs only change.

Release note: None