aws-eks: support pre-creating and supplying EKS Cluster OIDC Identity Provider to Cluster creation construct

### Describe the feature

The EKS Cluster creates an IAM identity Provider automatically as part of cluster provisioning.
I understand this only applies if a service account is added to the Cluster.
There is currently no way to pre-create this IdentityProvider and supply it to the cluster via a property in the construct

### Use Case

We have a customer business requirement for this from a security perspective, to pre-provision the IdentityProvider separately and pass this to the Cluster construction.
This allows removal of open access for Identity Provider creation across the Org.

### Proposed Solution

If possible, can we:
Add to ClusterProps interface: openIdConnectProvider: iam.IOpenIdConnectProvider
When populated, uses the supplied Identity Provider rather than creating its own during cluster creation.

### Other Information

_No response_

### Acknowledgements

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

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

2.234.1

### AWS CDK CLI version

2.1100.3

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

MacOS 26.2

Start at the aws-eks Cluster construct and its ClusterProps interface; trace where the IAM OIDC identity provider is created when service accounts are configured. Confirm the supplied iam.IOpenIdConnectProvider is used instead, and cover both supplied and omitted cases in the relevant construct tests.

Written by the indexing model from the issue text.