[Feature Request] Support for Client Certificates (mTLS)

[iDontWantAUsername]

Checklist

• [X] I have checked and there are no existing issues - open or closed - which request the same feature.
• [X] I have taken the time to fill in all the required details. I understand that the feature request will be dismissed otherwise.
• [X] This issue contains only one feature request.

Feature description

Now that the official NextCloud Android app supports Client Certificates it would be great if this app did too. This would mean when authenticating for the first time and a client certificate is required a system prompt would be show to select which already installed certificate to use. The certificate used will be remembered by the app and used for all future connections to the same URL.

Why do you want this feature?

Client Certificates are now supported by the NextCloud official Android app allowing an additional layer of security. This means that in order to access a NextCloud instance an additional certificate may be required that is installed on the device. This would allow access to passwords outside of my local network without a VPN via the app in the same way that the NextCloud app now works.

Additional information

NextCloud Android PR: https://github.com/nextcloud/android-library/pull/1308