ci: add copyright and license header check to CI runs

[jrasell]

HashiCorp legal requires that our source code files have copyright and license headers. There is a bot which runs periodically to checks this and will raise PR's for files which are missing the header. While this process works, it adds slight overhead in PR management. It could be nice to add a CI check which ensures any new files within the PR have the header.

[github-actions]

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