[FEATURE] Client Certificate authentication

[DeviaVir]

@RocketChat/android

Related to #1851

Disclaimer: I am very far removed from being an android or indeed mobile developer, please view this PR as a WIP, I am very open for feedback and changing this in a way that will make it something the Rocket.Chat team would like to merge and support, versus us keeping a local copy and merging in your changes every time.

Changes:

• added Select Client Certificate button
• added override interceptor
• added clientCertHelper and various links to this helper

It allows for the app to perform as usual with servers protected behind client certificates.

Known bugs:

• Requests to /websocket all still seem to fail due to not sending the correct certificate, though I'm unsure of the impact of this. The app seems to perform as usual.

Screenshots or GIF for the change:

(blurring is on purpose 😉)

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[nklock]

This is excellent news, our reverse proxy server requires users to us a client certificate as part of two factor authentication. This has worked well for web clients, but now we will be able to use mobile devices as well once this change is merged.

[DeviaVir]

@nklock I wouldn't bank on this actually making the cut. You can always generate and release your own app in-house, like we've done.