chore: add GitHub Actions workflow check for copyright header and fix missing copyright headers

[mattsb42-aws]

Description of changes:

This adds a GitHub Actions workflow for static analysis checks. Right now the only one is not-grep[1], which we're using to check for copyright headers in all source code files.

Also included are fixes for some files that we missed previously.

Run of workflow visible here[2].

[1] https://github.com/marketplace/actions/not-grep
[2] https://github.com/mattsb42-aws/aws-encryption-sdk-javascript/runs/686987225?check_suite_focus=true

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Check any applicable:

• Were any files moved? Moving files changes their URL, which breaks all hyperlinks to the files.