Support client certificates when connecting to Nextcloud

## Steps to reproduce
1.- Fresh install of Talk IOS.
2.- Put url "https://xxxx.xxxx.xxx/path/" from the Nextclout Server to do the login. (The url is really a reverse apache2 proxy to the nextcloud server),
3.- Push login button.

### Expected behaviour
The screen asking for the user and password must be shown.

### Actual behaviour
The url screen throws something like:

"Server Nextcloud not found.

The connection has been lost. Please, check that the url from server is correct." (It's a translation)

## Device information

**Device:** iPhone X
**iOS version:** ???

**Talk version:** v21.0.5

## Server information

**Nextcloud version:** v28.0.10
**Talk version:** v18.0.12
**Custom Signaling server configured:** no

**Custom TURN server configured:** yes
**Custom STUN server configured:** yes

### Server log (data/nextcloud.log)