aws-opensearchservice: Add OpenSearch custom package Resource and associated utils

### Name of the resource

Other

### Resource name

AWS:OpenSearchService

### Description

Customer reported this feature in the [github issue](https://github.com/aws/aws-cdk/issues/29710)

Proposed feature

Add support for instantiating OpenSearch custom package resources, usable when defining a Domain resource as well as for associating a package with existing domain resources. A CustomPackage Resource seems seems like a happy-path use case lacking simply pending further rollout of CDK support for OpenSearch

Use Case

Customer is looking to consistently use CDK to deploy/update infrastructure rather than have a console-cdk hybrid in some cases, which has to be the case for use of OpenSearch custom packages given the lack of CDK support at present.

Ticket created with OpenSearchService team - P125061378

Investigation so far

I checked the [Cloudformation docs](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-opensearchservice-domain.html) and it seems there is no L1 construct for the mentioned service. So I am willing to know if your team is planning to provide support on this feature in near future or is it already included in your roadmap?

### Other Details

_No response_