Add private key based authentication

[krzyk]

Please add private key based authentication (one that works e.g. from curl by passing in the --cert and --key options).

[markhatch]

I have been implementing this in a fork. I added mtls cert support to local installs. I can try to open a PR with the feature, but I likely will need to clean it up a bit.

[markhatch]

@ankitpokhrel would you be open to that? Essentially I add a authType mtls which then prompts users to enter CA Cert + Client Certificates file locations, which get used instead of basic/bearer auth. It works on my on-prem server.

[ankitpokhrel]

Hey, would absolutely love and appreciate your pull request! :)

[markhatch]

Took a pass at implementation here: #615