This change introduces an `mi2`-like experience for interacting with MSP databases. By default we use the new readonly-SA introduced in https://github.com/sourcegraph/sourcegraph/pull/59105, otherwise with the `--write-access` flag you can connect as the same user as the Cloud Run workload, which gives a lot more permissions.
An alias, `sg msp pg connect`, is available for those less inclined to type out the entire `postgresql` subcommand name.
Closes https://github.com/sourcegraph/managed-services/issues/207
## Test plan
Applied to `msp-testbed`, which has a PG instance and provisioned tables.
Try both service accounts:
```
sg msp pg connect --write-access msp-testbed test
sg msp pg connect msp-testbed test
```
With both of the above:
```
primary=> select * from users;
id | created_at | updated_at | deleted_at | external_id | name | avatar_url
----+------------+------------+------------+-------------+------+------------
(0 rows)
```
