fix(rds): unable to use tokens as port in DatabaseInstance (aws#17995)

In `DatabaseInstance` the port number can be specified using data type number.
If a token value (e.g. CloudFormation Parameter) was used here, the token was not resolved correctly.

The conversion of property `port` has been corrected by using method `Tokenization.stringifyNumber()`.

Fixes aws#17948.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-rds/lib/instance.ts

@@ -746,7 +746,7 @@ abstract class DatabaseInstanceNew extends DatabaseInstanceBase implements IData
       performanceInsightsRetentionPeriod: enablePerformanceInsights
         ? (props.performanceInsightRetention || PerformanceInsightRetention.DEFAULT)
         : undefined,
-      port: props.port?.toString(),
+      port: props.port !== undefined ? Tokenization.stringifyNumber(props.port) : undefined,
       preferredBackupWindow: props.preferredBackupWindow,
       preferredMaintenanceWindow: props.preferredMaintenanceWindow,
       processorFeatures: props.processorFeatures && renderProcessorFeatures(props.processorFeatures),

packages/@aws-cdk/aws-rds/test/instance.test.ts

@@ -1630,6 +1630,43 @@ describe('instance', () => {
       },
     });
   });
+
+  test('instance with port provided as a number', () => {
+    // WHEN
+    new rds.DatabaseInstance(stack, 'Database', {
+      engine: rds.DatabaseInstanceEngine.MYSQL,
+      instanceType: ec2.InstanceType.of(ec2.InstanceClass.BURSTABLE2, ec2.InstanceSize.SMALL),
+      vpc,
+      port: 3306,
+    });
+
+    // THEN
+    expect(stack).toHaveResourceLike('AWS::RDS::DBInstance', {
+      Port: '3306',
+    });
+  });
+
+  test('instance with port provided as a CloudFormation parameter', () => {
+    // GIVEN
+    const port = new cdk.CfnParameter(stack, 'Port', {
+      type: 'Number',
+    }).valueAsNumber;
+
+    // WHEN
+    new rds.DatabaseInstance(stack, 'Database', {
+      engine: rds.DatabaseInstanceEngine.MYSQL,
+      instanceType: ec2.InstanceType.of(ec2.InstanceClass.BURSTABLE2, ec2.InstanceSize.SMALL),
+      vpc,
+      port,
+    });
+
+    // THEN
+    expect(stack).toHaveResourceLike('AWS::RDS::DBInstance', {
+      Port: {
+        Ref: 'Port',
+      },
+    });
+  });
 });
 
 test.each([