mod_rest/rest.sh: Update to use httpie-oauth2 plugin This bash implementation of OAuth2/OIDC was growing to the point where it needed a massive refactor, which made me look into alternatives where I finally settled on implementing oauth2 in a plugin for HTTPie.

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summary: Log a message once per minute
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This module sends `-- MARK --` to the log once per minute. This may be
useful to give a sense of how busy the server is or see that logging and
timers are still working.