Add CockroachDB support

[r0mant]

CockroachDB uses Postgres wire protocol so it's pretty much supported out-of-the-box (with client certificate auth) but this PR implements a few updates for better integration and to account for some differences in auth:

• Update tctl auth sign command to support --format=cockroachdb which produces secrets in Cockroach-friendly format. That includes filenames (Cockroach expects them to be node.crt, node.key, ca.crt) and mandatory CN=node for the node certs. See their docs for more details if interested.

• Add protocol: cockroachdb for database configuration. Technically this is the same as protocol: postgres on the backend, but it is used e.g. to pick appropriate client to connect with.

• Related to the above, update tsh db connect to use native cockroach CLI client to connect to CockroachDB. It falls back to psql (which is compatible) if cockroach isn't found.

• For authorization, do not use db_names for RBAC. Only db_users are enforced, similar to MySQL. This is because, unlike Postgres, in Cockroach you can switch database at any time without reconnecting and cross-database queries are allowed so limiting db_names on Teleport side would be pointless.

The guide will be coming in a separate PR.

[nklaassen]

LGTM

[russjones]

Bot.