Security: Add mTLS client certificate support

[paolostivanin]

Allow users to present a client certificate for mutual TLS authentication (e.g. Cloudflare mTLS). Uses Android KeyChain API so users pick from certificates already installed on device.

Tested on my device (OnePlus 13R) with Android 16. My setup is: Cloudflare (with mTLS) -> NPM -> homelab

I've installed the opencloudApp/build/outputs/apk/qa/debug/OpenCloud_1.2.1-qa-debug.apk on my device and with this is version and the certificate selected I can browse my data, with upstream's version sync and browsing fail (as expected).

I've been using this debug apk for a few days now and I have not faced any issues.

Please note that I am NOT an Android developer, so please review the changes carefully for any mistake I might have made 😃

[guruz]

Thank you for the PR, looks small enough to merge.

However: Did you see @michaelstingl 's proposal here: https://github.com/orgs/opencloud-eu/discussions/819#discussioncomment-13102292

[paolostivanin]

hello!

Yes, I saw that discussion but as opencloud-eu/opencloud#819 (reply in thread) pointed out, this is a different use case compared to the IdP mTLS 😃

[paolostivanin]

hello :) is there any more feedback about this PR? Thanks

[guruz]

@paolostivanin I will have a look for 1.3.0 :)

[paolostivanin]

@paolostivanin I will have a look for 1.3.0 :)

thanks a lot, much appreciated 😄

[GIT-MarkBowman]

Built and sideloaded on my Samsung Galaxy S22 on Android 16; using CF Zero Trust and tunnel --> caddy --> Opencloud with Authentik OIDC. I'll post any issues I have.

[guruz]

@paolostivanin Thank you, what are the changes from before?

[paolostivanin]

@paolostivanin Thank you, what are the changes from before?

Just a rebase against latest master, I saw that there was a merge conflict.