Add support for authorizing connections based on public key

[centromere]

#213 no longer merges cleanly. This PR provides all the functionality contributed by @mbutkereit, as well as relevant changes to the Helm chart to support authorized keys.

[owenthereal]

Thanks for the contribution! Can we add a test, please?

[centromere]

Hi @owenthereal. I have began writing a unit test, but I am struggling to figure out the best way to wire it in with the rest of the code base. Any suggestions you have would be much appreciated.