feat(opensearchservice): add useLatestServiceSoftwareForBlueGreen support

[Swathi-Karanth]

Issue

Adds support for UseLatestServiceSoftwareForBlueGreen in the OpenSearch domain SoftwareUpdateOptions, aligning the L2 construct with the service API.

Reason for this change

The OpenSearch Service API supports UseLatestServiceSoftwareForBlueGreen in SoftwareUpdateOptions, which allows domains to automatically use the latest available service software when a blue/green deployment is triggered. This property is already in the CloudFormation resource schema (AWS::OpenSearchService::Domain) but was missing from the CDK L2 construct.

Description of changes

• Added useLatestServiceSoftwareForBlueGreen optional boolean prop to DomainProps
• Updated L1 wiring to pass the property through to softwareUpdateOptions
• Updated the softwareUpdateOptions condition to render when either enableAutoSoftwareUpdate or useLatestServiceSoftwareForBlueGreen is specified
• Updated README with usage example
• Added unit tests

Description of how you validated changes

Unit tests covering:

• useLatestServiceSoftwareForBlueGreen set to true
• useLatestServiceSoftwareForBlueGreen set to false
• Both props used together
• Existing tests still pass (absent when neither prop specified)

Checklist

• My code adheres to the CONTRIBUTING GUIDE
• Is this a breaking change (changing existing behavior or interface)? No
• Does this change require a documentation update? Yes (README updated)
• Does this change require an update to the integration tests? Not required (no new CFN resource type)

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[github-actions]

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[aws-cdk-automation]

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