(aws-opensearchservice): support DeploymentStrategyOptions on Domain

### Describe the feature

Expose `DeploymentStrategyOptions` on the L2 `Domain` construct in `aws-cdk-lib/aws-opensearchservice`. Allowed values: `Default`, `CapacityOptimized`.

### Use Case

Users want to configure capacity-optimized deployments at the L2 level without falling back to `defaultChild` escape hatches.

### Proposed Solution

Add an optional `deploymentStrategy?: DeploymentStrategy` prop on `DomainProps` and wire it through to `CfnDomain.deploymentStrategyOptions`.

### Other Information

L1 (`CfnDomain`) already supports this property.

### 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)

2.257.0

### AWS CDK CLI version

2.1123.0

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

macOS

Start at the L2 Domain construct and its DomainProps definition, then inspect how properties are wired to the L1 CfnDomain. Use the existing CfnDomain deploymentStrategyOptions support as the reference; done means an optional DeploymentStrategy prop accepts Default or CapacityOptimized and reaches the L1 construct without requiring defaultChild escape hatches.

Written by the indexing model from the issue text.