fix(aws-lambda): jsii use no long requires 'sourceAccount' (#728)

Change the typing of 'sourceAccount' to be optional string, so that the
typing over JSII will be properly inferred. The current type is 'any' so
that we can pass in Tokens, but since the Token stringification change
typing as a string no longer forms an obstacle, and in fact it will be
more inline with upcoming type system changes.

Fixed #714.

Author: rix0rrr

packages/@aws-cdk/aws-lambda/lib/lambda-ref.ts

@@ -317,7 +317,7 @@ export abstract class FunctionRef extends cdk.Construct
         const permissionId = `AllowBucketNotificationsFrom${bucketId}`;
         if (!this.tryFindChild(permissionId)) {
             this.addPermission(permissionId, {
-                sourceAccount: new cdk.AwsAccountId(),
+                sourceAccount: new cdk.AwsAccountId().toString(),
                 principal: new cdk.ServicePrincipal('s3.amazonaws.com'),
                 sourceArn: bucketArn,
             });

packages/@aws-cdk/aws-lambda/lib/permission.ts

@@ -42,7 +42,7 @@ export interface Permission {
      * bucket owner's account ID. You can use this property to ensure that all
      * source principals are owned by a specific account.
      */
-    sourceAccount?: any;
+    sourceAccount?: string;
 
     /**
      * The ARN of a resource that is invoking your function. When granting

packages/@aws-cdk/aws-lambda/test/test.lambda.ts

@@ -118,7 +118,7 @@ export = {
             fn.addPermission('S3Permission', {
                 action: 'lambda:*',
                 principal: new cdk.ServicePrincipal('s3.amazonaws.com'),
-                sourceAccount: new cdk.AwsAccountId(),
+                sourceAccount: new cdk.AwsAccountId().toString(),
                 sourceArn: new cdk.Arn('arn:aws:s3:::my_bucket')
             });