(aws_elasticloadbalancingv2.ApplicationListener): SslPolicy change to allow both SslPolicy Enum and Str

### Describe the feature

in the past few years I quite often have new security compliance guideline to require us to update the SSL policy on load balancer, however the CDK release schedule simply cannot catch up with how the security guideline release speed. (in this case, we are requested to update to use ELBSecurityPolicy-TLS13-1-2-RFC9151-FIPS-2023-07 this month, it was only released 3 weeks ago)

Currently ApplicationListener SslPolicy only accept the enum object, and I can't use latest SslPolicy without some hack around the base class property override.

### Use Case

Allow us freely use any SSL policy supported by AWS instead restrict by SslPolicy enum.

### Proposed Solution

Allow sslPolicy field accept string

```
elbv2.ApplicationListener(
self.scope,
"Listener",
ssl_policy='ELBSecurityPolicy-TLS13-1-2-RFC9151-FIPS-2023-07'
)

# instead of
listener.node.default_child.add_property_override('SslPolicy', 'ELBSecurityPolicy-TLS13-1-2-RFC9151-FIPS-2023-07')
```

### Other Information

https://docs.aws.amazon.com/elasticloadbalancing/latest/application/describe-ssl-policies.html#rfc9151-security-policies
https://aws.amazon.com/about-aws/whats-new/2026/08/aws-application-network/

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### AWS CDK Library version (aws-cdk-lib)

2.266.0

### AWS CDK CLI version

2.1135.1

### Environment details (OS name and version, etc.)

Linux

Start at the aws_elasticloadbalancingv2 ApplicationListener entry point and inspect how its SslPolicy property currently accepts the SslPolicy enum. Check the related module tests before changing the input contract, then verify that both enum values and arbitrary AWS SSL policy strings synthesize the requested SslPolicy value.

Written by the indexing model from the issue text.