sql: type string literals as `unknown` initially

In Postgres, string literals (e.g., in `SELECT 'foo'`) are typed as `unknown` initially. It's type is then resolved later on based on type conversion rules of function calls, operators, and other constructs.

More information on string literal typing in Postgres can be found in the docs: https://www.postgresql.org/docs/current/typeconv-overview.html

Epic CRDB-2474

Jira issue: CRDB-23099