feat(crdb): cusomize CockroachDB connection

feat(crdb): cusomize CockroachDB connection

- Add ConnectionInfo.vendor
- Add ConnectionInfo subclassing
- Add CrdbConnectionInfo subclass with crdb_version attribute
- Add 'psycopg.crdb' module with adapters
- Dump strings using the text oid by default on CockroachDB

The latter change might have wider consequences, however crdb casts
strings to other types more easily than what Postgres does. At a glance
it might work; porting the rest of the test suite will tell.

test_adapt tests ported. Tests showing a difference moved to
crdb/test_adapt.