fix(aws-route53-targets): add support for custom cname_prefix urls in elastic beanstalk environment endpoint target (#18804)

This PR fixes the extraction of the region name from a Elastic Beanstalk Environment URL generated with a custom CNAME_PREFIX.

The code is backward compatible with the regular URL

# Motivation

ElasticBeanstalkEnvironmentEndpointTarget is used to create an alias target for ElasticBeanstalk Environments that are already created and published for a certain region.

When creating an ElasticBeanstalk Environment is possible to configure a `cname_prefix` in order to have a "deterministic" url for the generated environment.

Normally the generated url looks like `mybeanstalkenvironment.xyz.eu-west-1.elasticbeanstalk.com`, however when the `cname_prefix` is specified the url loses the randomly generated hash and looks like `mycnameprefix.eu-west-1.elasticbeanstalk.com`.

In the custom `cname_prefix` scenario the `ElasticBeanstalkEnvironmentEndpointTarget` class fails with the following error:
```
jsii.errors.JSIIError: Elastic Beanstalk environment target is not supported for the "elasticbeanstalk" region.
```

I mentioned this problem also [here](#17992 (comment)), sorry for the double comment. This PR does not fix the original problem of that issue thread.

# How to reproduce
Consider this scenario:
```
# app1.py
# EB Enviroment
enviroment = elasticbeanstalk.CfnEnvironment(self, 'Enviroment', 
    environment_name='MySampleEnviroment',
    application_name=application.application_name or application_name,
    solution_stack_name=SOLUTION_STACK_NAME,
    option_settings=self.get_option_setting_properties(),
    version_label=app_version_props.ref,
    cname_prefix='myapp'
)
```

This generates an elastic beanstalk environment with url `myapp.eu-west-1.elasticbeanstalk.com` (supposing one it's deploying in eu-west-1).

Then a following cdk app tries to create a DNS record for it
```
# app2.py
route53.ARecord(self, f'{construct_label}AliasRecord',
    target=route53.RecordTarget.from_alias(
        targets.ElasticBeanstalkEnvironmentEndpointTarget(     
            'myapp.eu-west-1.elasticbeanstalk.com',
        ),
    ),
    zone=domain_configuration.hosted_zone,
)
```

At this point the command cdk diff returns the following error:
```
jsii.errors.JSIIError: Elastic Beanstalk environment target is not supported for the "elasticbeanstalk" region.
```

----

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

Author: nicfix

packages/@aws-cdk/aws-route53-targets/lib/elastic-beanstalk-environment-target.ts

@@ -5,6 +5,7 @@ import { RegionInfo } from '@aws-cdk/region-info';
 /**
  * Use an Elastic Beanstalk environment URL as an alias record target.
  * E.g. mysampleenvironment.xyz.us-east-1.elasticbeanstalk.com
+ * or mycustomcnameprefix.us-east-1.elasticbeanstalk.com
  *
  * Only supports Elastic Beanstalk environments created after 2016 that have a regional endpoint.
  */
@@ -18,7 +19,9 @@ export class ElasticBeanstalkEnvironmentEndpointTarget implements route53.IAlias
     }
 
     const dnsName = this.environmentEndpoint;
-    const region = cdk.Fn.select(2, cdk.Fn.split('.', dnsName));
+    const subDomains = cdk.Fn.split('.', dnsName);
+    const regionSubdomainIndex = subDomains.length - 3;
+    const region = cdk.Fn.select(regionSubdomainIndex, subDomains);
     const { ebsEnvEndpointHostedZoneId: hostedZoneId } = RegionInfo.get(region);
 
     if (!hostedZoneId || !dnsName) {

packages/@aws-cdk/aws-route53-targets/test/elastic-beanstalk-environment-target.test.ts

@@ -23,3 +23,25 @@ test('use EBS environment as record target', () => {
     },
   });
 });
+
+
+test('support 4-levels subdomain URLs for EBS environments', () => {
+  // GIVEN
+  const stack = new Stack();
+  const zone = new route53.PublicHostedZone(stack, 'HostedZone', { zoneName: 'test.public' });
+
+  // WHEN
+  new route53.ARecord(stack, 'Alias', {
+    zone,
+    recordName: '_foo',
+    target: route53.RecordTarget.fromAlias(new targets.ElasticBeanstalkEnvironmentEndpointTarget('mycustomcnameprefix.us-east-1.elasticbeanstalk.com')),
+  });
+
+  // THEN
+  Template.fromStack(stack).hasResourceProperties('AWS::Route53::RecordSet', {
+    AliasTarget: {
+      DNSName: 'mycustomcnameprefix.us-east-1.elasticbeanstalk.com',
+      HostedZoneId: 'Z117KPS5GTRQ2G',
+    },
+  });
+});