Add support for external collaborators to peribolos

[deepsm007]

GitHub supports external collaborators to provide permissions to individuals outside of an organization: https://docs.github.com/en/organizations/managing-user-access-to-your-organizations-repositories/managing-outside-collaborators/adding-outside-collaborators-to-repositories-in-your-organization

Peribolos does not presently support managing external collaborators. Add external collaborator support to peribolos

As is typical for peribolos, if the value is not specified, peribolos should not attempt to manage it. This should make the feature backwards compatible. It will be necessary to set "external_collaborators: []" to clear out any old external collaborators.