mysql_clear_password can have string type

- mysql_clear_password in authPlugins currently only supports
Promise<string> type. However AWS.RDS.Signer.getAuthToken returns string, which
causes type error "Type 'string' is not assignable"
- This fix will add support for string type as well

Author: tomoima525

index.d.ts

@@ -151,9 +151,10 @@ export interface Pool extends mysql.Connection {
   promise(promiseImpl?: PromiseConstructor): PromisePool;
 }
 
-type authPlugins =
-    (pluginMetadata: { connection: Connection; command: string }) =>
-        (pluginData: Buffer) => Promise<string>;
+type authPlugins = (pluginMetadata: {
+  connection: Connection;
+  command: string;
+}) => (pluginData: Buffer) => Promise<string> | string;
 
 export interface ConnectionOptions extends mysql.ConnectionOptions {
   charsetNumber?: number;
@@ -175,7 +176,7 @@ export interface ConnectionOptions extends mysql.ConnectionOptions {
   queueLimit?: number;
   waitForConnections?: boolean;
   authPlugins?: {
-      [key: string]: authPlugins;
+    [key: string]: authPlugins;
   };
 }