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.

Written by the indexing model from the issue text.