feat(coderd/database): add value_key_id column to user_secrets for encryption (#23997)

Add a nullable `value_key_id` column to the `user_secrets` table with a
foreign key to `dbcrypt_keys`. This is the column dbcrypt uses to track
which encryption key encrypted a given secret's value. This is required
for encryption of user secret values.

The column was missing from the original migration (000357).

coderd/database/migrations/000460_user_secrets_value_key_id.down.sql

@@ -0,0 +1,3 @@
+ALTER TABLE user_secrets
+    DROP CONSTRAINT user_secrets_value_key_id_fkey,
+    DROP COLUMN value_key_id;

coderd/database/migrations/000460_user_secrets_value_key_id.up.sql

@@ -0,0 +1,5 @@
+ALTER TABLE user_secrets
+    ADD COLUMN value_key_id TEXT;
+
+ALTER TABLE ONLY user_secrets
+    ADD CONSTRAINT user_secrets_value_key_id_fkey FOREIGN KEY (value_key_id) REFERENCES dbcrypt_keys(active_key_digest);