Add support for customizing tag keys

[knziiy]

Description of Changes

• Add tagKey parameter: Added a new optional parameter.
• Update tag application logic: Updated the main CDK app and the RAG Knowledge Base stack to use the configurable tag key.

Checklist

• Modified relevant documentation
• Verified operation in local environment
• Executed npm run cdk:test and if there are snapshot differences, execute npm run cdk:test:update-snapshot to update snapshots

Related Issues

• タグのキーに任意の値を指定したい #1325

Note

Validation for the tagKey input value is not performed in this implementation.
This is because tagging specifications differ across AWS services, and we also do not currently validate the existing tagValue.
Instead, we rely on the validation checks performed by each respective AWS service during deployment.