… elastic beanstalk environment endpoint target (aws#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](aws#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*