Add license headers and related checks

The LICENSE file is missed currently.

Before moving the C++ client from Pulsar main repo here, the license check was performed by `mvn license:check`. After adding the LICENSE file, we should add a license verification workflow.

Start by inspecting the repository for the missing LICENSE file, existing license headers, and workflow configuration. Review how the former `mvn license:check` check applied to the C++ client and determine the equivalent verification scope. Done means the LICENSE file and required headers are present and an automated workflow verifies them.

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