chore: implement databased backend for custom roles (#13295)

Includes db schema and dbauthz layer for upserting custom roles. Unit test in `customroles_test.go` verify against escalating permissions through this feature.

coderd/database/migrations/000209_custom_roles.down.sql

@@ -0,0 +1,2 @@
+DROP INDEX IF EXISTS idx_custom_roles_name_lower;
+DROP TABLE IF EXISTS custom_roles;