Internal SDK Differentiator

[cb-karthikp]

• Add InternalChargebeeClient generation to differentiate the internal SDK's
  User-Agent from the public SDK for Splunk observability.
  The internal client requires
  a mandatory serviceName parameter and sends Chargebee-Java-Internal-Client
  v{version}-{serviceName} as the User-Agent header.
• Fix batch requests to propagate clientHeaders so batch operations also use the
  custom User-Agent.

[snyk-io]

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