Add HMAC protection on /approval endpoint

Signed-off-by: Bob Callaway <bcallaway@google.com>

storage/ent/schema/authrequest.go

@@ -27,7 +27,8 @@ create table auth_request
     expiry                    timestamp not null,
     claims_preferred_username text default '' not null,
     code_challenge            text default '' not null,
-    code_challenge_method     text default '' not null
+    code_challenge_method     text default '' not null,
+    hmac_key                  blob
 );
 */
 
@@ -86,6 +87,7 @@ func (AuthRequest) Fields() []ent.Field {
 		field.Text("code_challenge_method").
 			SchemaType(textSchema).
 			Default(""),
+		field.Bytes("hmac_key"),
 	}
 }