[admin] GHA workflow

[rustielin]

After #93, we should set up a demo/test repo to submit traversal states against. A key part of this will be using GHA to sign ITS on issue actions.

Ideally, we can sign the ITS when issue has been closed by an admin of the repo.

• https://help.github.com/en/rest/reference/issues
• https://help.github.com/en/actions/reference/events-that-trigger-workflows

[rustielin]

Initial workflow done and merged in https://github.com/indexofknowledge/iok-admin, but next steps are to allow import of a signing key into GHA and also write a GHA workflow that runs when admin issues a certain command/comment