Add new flyctl mpg credentials for convenience

[soulware]

Add flyctl mpg credentials for convenience.
This outputs the connection URI for the specified MPG cluster.

./bin/flyctl mpg credentials --cluster zp2wjregg49odn4q
postgres://fly-user:<REDACTED>@pgbouncer.zp2wjregg49odn4q.flympg.net/fly-db

./bin/flyctl mpg credentials
? Select Organization: [...]
? Select a Postgres cluster [...]
postgres://fly-user:<REDACTED>@pgbouncer.zp2wjregg49odn4q.flympg.net/fly-db

./bin/flyctl mpg credentials --org <ORG_SLUG>
? Select a Postgres cluster [...]
postgres://fly-user:<REDACTED>@pgbouncer.zp2wjregg49odn4q.flympg.net/fly-db