feat: add custom name support to RDS OptionGroup

[nehaaprasad]

Closes #35720.

Reason for this change

AWS CDK lacked ability to set custom names for RDS option groups despite CloudFormation support.

Description of changes

• Added optionGroupName?: string to OptionGroupProps
• Updated constructor and CloudFormation resource to use custom name
• Added test coverage and README documentation

Description of how you validated changes

• Added unit test for custom name functionality
• Verified backward compatibility

Checklist

• Follows CDK contribution guidelines

[aws-cdk-automation]

The pull request linter fails with the following errors:

❌ Features must contain a change to an integration test file and the resulting snapshot.

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[aws-cdk-automation]

This PR has been in the CHANGES REQUESTED state for 3 weeks, and looks abandoned. Note that PRs with failing linting check or builds are not reviewed, please ensure your build is passing

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[aws-cdk-automation]

This PR has been in the CHANGES REQUESTED state for 3 weeks, and looks abandoned. Note that PRs with failing linting check or builds are not reviewed, please ensure your build is passing

To prevent automatic closure:

• Resume work on the PR
• OR request an exemption by adding a comment containing 'Exemption Request' with justification e.x "Exemption Request: "
• OR request clarification by adding a comment containing 'Clarification Request' with a question e.x "Clarification Request: "

This PR will automatically close in 14 days if no action is taken.

[aws-cdk-automation]

This PR has been deemed to be abandoned, and will be automatically closed. Please create a new PR for these changes if you think this decision has been made in error.

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