Fix types for callback argument in get & refresh methods for Cr… (#3127)

Co-authored-by: Sameer Goyal <sameeg@amazon.com>

.changes/next-release/bugfix-Credentials-08f473f2.json

@@ -0,0 +1,5 @@
+{
+  "type": "bugfix",
+  "category": "Credentials",
+  "description": "Fix types for callback argument in get & refresh methods for Credentials to accept error optionally"
+}

lib/credentials.d.ts

@@ -21,7 +21,7 @@ export class Credentials {
      *
      * @param {get} callback - Called when the instance metadata service responds. When called with no error, the credentials information has been loaded into the object.
      */
-    get(callback: (err: AWSError) => void): void;
+    get(callback: (err?: AWSError) => void): void;
     /**
      * Gets the existing credentials, refreshing them if necessary, and returns
      * a promise that will be fulfilled immediately (if no refresh is necessary)
@@ -38,7 +38,7 @@ export class Credentials {
      *
      * @param {function} callback - Called when the instance metadata service responds. When called with no error, the credentials information has been loaded into the object.
      */
-    refresh(callback: (err: AWSError) => void): void;
+    refresh(callback: (err?: AWSError) => void): void;
     /**
      * Invokes a credential refresh and returns a promise that will be fulfilled
      * when the refresh has completed or rejected when the refresh has failed.