Support the creation of logical SQL schemas

[jseldess]

Background: https://cockroachlabs.atlassian.net/browse/CRDB-418

Description: Introduce the distinction between database and schema in namespaces. As a result, developers can specify different logical SQL schemas. In addition, many tools will remove an important blocker to working out-of-the-box with CRDB.

Complex applications organize their databases and table schemas in different groups. Currently, Cockroach offers the option to use multiple databases within one cluster and only provides one pre-defined physical schema "public" in every database.

Many other databases like Postgres offer multiple schemas. This means that many PG applications and third-party tooling such as SQLAlchemy, Hibernate, and PGAdmin expect CockroachDB to also offer this capability. Without multiple schemas, these applications and tools will not work "out of the box."

Multiple schemas can also impact our ability to support multi-tenancy. A more complex permissions model might necessitate this feature to distinguish the database and schema from each tenant .

Similarly to multi-tenancy, this feature will also increase the ability for CockroachDB to support multiple microservices within one CockroachDB cluster.

Team: Andrew Woods, Richard Loveland, Jordan Lewis

Github Tracking Issue: cockroachdb/cockroach#26443 cockroachdb/cockroach#30916 https://github.com/cockroachlabs/support/issues/299#issuecomment-485923876 #4787

[vy-ton]

We will want to document, cockroachdb/cockroach#52997, to help users convert a database to a schema