feat: Make db-uri optional

The default is now "postgresql://" which falls back to LIBPQ environment variables.

Resolves #1991, Ref #1823

test/io/configs/expected/no-defaults-with-db-other-authenticator.config

@@ -11,7 +11,7 @@ db-root-spec = "other_root"
 db-schemas = "test,other_tenant1,other_tenant2"
 db-config = true
 db-tx-end = "rollback-allow-override"
-db-uri = "<REPLACED_WITH_DB_URI>"
+db-uri = "postgresql://"
 db-use-legacy-gucs = false
 jwt-aud = "https://otherexample.org"
 jwt-role-claim-key = ".\"other\".\"role\""