[feature request] Client certificate support for TLS authentication

[lescisin]

Hi there - Thanks for all of the excellent work on this project. Really great stuff - I've only been using it for about a week and already it's been helpful for me. I especially appreciate the simplicity of posting messages - it makes it so easy to integrate into other services.

My suggestion:

I was hoping to use client certificates on my front end proxy for TLS authentication to ntfy. I configured my proxy for this and I'm able to authenticate and then access ntfy via browser - I get prompted for the client cert and all works. (hooray!) However, doing the same on the android app, I get the error below.

I'm thinking that the client Android app does not support client certificates based on this. I am submitting this suggestion in hopes that the feature might be considered as a future enhancement.