Element R: fix `isCrossSigningReady` not checking identity trust

[BillCarsonFr]

Fix an inconsistency between legacy crypto and rust crypto when calling isCrossSigningReady. Legacy is checking that cross-signing is set up and that the keys are trusted. Rust was just checking that cross-signing was set up.

Related issue https://github.com/element-hq/customer-retainer/issues/224

Checklist

• [x] Tests written for new code (and old code if feasible).
• [ ] New or updated public/exported symbols have accurate TSDoc documentation.
• [x] Linter and other CI checks pass.
• [ ] Sign-off given on the changes (see CONTRIBUTING.md).

[richvdh]

@BillCarsonFr It would be good to summarise what effects this bug might have on the user experience. IIRC we have at least one bug report which we believe was caused by this? But it doesn't seem to be linked.

[BillCarsonFr]

@BillCarsonFr It would be good to summarise what effects this bug might have on the user experience. IIRC we have at least one bug report which we believe was caused by this? But it doesn't seem to be linked.

added in description