Support specifying a client certificate for mTLS auth

[tinsukE]

Using a reverse-proxy with mTLS (client authentication) is a decent way to expose your self-hosted services on the internet.

This PR adds support for using a certificate that's installed in your Android device.

The changes are largely based on:

• This Photoprism client: mTLS auth Radiokot/photoprism-android-client#14
• This sample: https://github.com/diebietse/mtls-android

An optional client certificate can be selected when adding self-hosted accounts (FreshRSS, Google Reader and Fever APIs):

It can later be changed/removed when viewing the account details:

I'm comfortable with the logic changes, but I have not much experience with Compose, so my code there can be sub-optimal.

[Ashinch]

This looks good.