Add support for device dehydration v2 (Element R)

[uhoreg]

Depends on https://github.com/matrix-org/matrix-rust-sdk-crypto-wasm/pull/104 (so tests are expected to fail)

Checklist

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

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Add one of: T-Deprecation, T-Enhancement, T-Defect, T-Task to indicate what type of change this is plus X-Breaking-Change if it's a breaking change.