[WIP] FOSS infrastructure for X.509 signing and verification

[kaylendog]

Content

This PR implements the Android-native side of the X.509 signing and verification work, introducing a signer and verifier implementation, as well as a startup hook to look for keychains on-device.

Depends on matrix-org/matrix-rust-sdk#6727 and a release of the Kotlin components, as well as the companion enterprise PR.

Motivation and context

X.509 verification allows devices to be loaded with homeserver-origin certificates, and key chains signed by said certificate, allowing the device to be automatically verified.

This PR is based on the work by @andybalaam and @richvdh on the andyrich/x509 branch and should be nearly identical, bar a few nits and a tidier commit history.

Screenshots / GIFs

WIP

Tests

• Step 1
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Tested devices

• Physical
• Emulator
• OS version(s): Android 36.1

Checklist

• I am aware of the etiquette.
• This PR was made with the help of AI:
  • Yes. In this case, please request a review by Copilot.
  • No.
• Changes have been tested on an Android device or Android emulator with API 24
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request title will be used in the release note, it clearly defines what will change for the user
• Pull request includes screenshots or videos if containing UI changes
• You've made a self review of your PR

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Generated by 🚫 dangerJS against 942e62c

[sonarqubecloud]

Quality Gate passed

Issues
6 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud