Add github.com/elastic/go-licenser

per @felixbarny 

> The Go agent uses a utility to automatically add license headers during build. Seems sensible to add it to this project, too. https://github.com/elastic/apm-agent-go/blob/36bdfef17b747138149f3a26c1ee46c1f7eb5207/Makefile#L26-L27

This issue represents the work needed to add this feature to this repository. 
