Add license header checking app

[markmcd]

Almost the same config as the generative-ai-docs repo: https://github.com/google/generative-ai-docs/blob/main/.github/header-checker-lint.yml

[Giom-V]

Approved, but have you run it on the existing .py files? Do we need to update their licences?

[markmcd]

Approved, but have you run it on the existing .py files? Do we need to update their licences?

Yeah, they will need to be updated too. Typically I would do the "prevent backsliding" step before the "update the world" step, since they're independent, but it looks like there's only the file-api samples so I'll update it here.