Add option to persist security key on platform

[Nils1729]

This PR adds a feature_persist_ssss_key feature flag. When enabled, element-desktop tries to save any generated security key in the login keyring, similar to pickle keys. It automatically uses them for unlocking the backup. See also https://github.com/vector-im/element-web/pull/26405 and https://github.com/vector-im/element-desktop/pull/1286.

Goals

With this PR, we try to work towards user-friendly encryption key backups while preserving e2ee security. In our small/medium non-profit org, many members lack a technical background. Using element as-is without losing keys would be quite a challenge for them.

Since we might not be the only org struggling with secure backup/key management in practice (https://github.com/vector-im/element-web/issues/20046), we would like to contribute to Element.

This PR and the whole feature are experimental and we would love to receive comments or feedback.

Checklist

• [x] Tests written for new code (and old code if feasible)
• [ ] Linter and other CI checks pass
• [x] Sign-off given on the changes (see CONTRIBUTING.md)

Notes: none element-web notes: none element-desktop notes: Add option to persist security key

Type: enhancement

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This change has no change notes, so will not be included in the changelog.