aws-eks: AutoScalingGroupCapacityOptions - support MixedInstancesPolicy

### Describe the feature

Add MixedInstancesPolicy as an optional parameter to the AutoScalingGroupCapacityOptions interface. Update validation logic to address incompatible arguments.

### Use Case

I am unable to specify multiple instance types for my EKS self managed capacity created via `cluster.addAutoScalingGroupCapacity`. This can be accomplished by specifying multiple instance types in a `MixedInstancesPolicy`.

### Proposed Solution

Add a new optional parameter `MixedInstancesPolicy` to the `AutoScalingGroupCapacityOptions` interface. Make `instanceType` an optional field. Adjust validation logic to handle cases where certain fields cannot be specified alongside `MixedInstancesPolicy`. The validation will be similar to what is done in the EKS `NodeGroup` constructor https://github.com/aws/aws-cdk/blob/v2.219.0/packages/aws-cdk-lib/aws-eks/lib/managed-nodegroup.ts#L389

### 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)

aws-cdk-lib@2.215.0

### AWS CDK CLI version

2.1030.0

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

MacOS Sequoia 15.7.1

Start with cluster.addAutoScalingGroupCapacity and the AutoScalingGroupCapacityOptions interface, then compare its validation with the EKS NodeGroup constructor in packages/aws-cdk-lib/aws-eks/lib/managed-nodegroup.ts around line 389. Done means MixedInstancesPolicy is accepted optionally, instanceType is optional, and incompatible arguments are rejected by validation.

Written by the indexing model from the issue text.