Add tooling for checking copyright headers.

[arteevraina]

Currently, we do not have any tooling for checking the copyright headers in the dart files of the apps in the repo which results in accidentally pushing the code without copyright headers. As suggested by @johnpryan, we can check this using GitHub Actions with the help of this package.

Did I miss something @ditman @johnpryan?

[ditman]

Sounds good to me. If this gets done, I'd love to copy this to other flutter repos! I think this'd be a fantastic addition to flutter/plugins, at least!