chore(changelog): add MSC3814 dehydrated-devices fragments.

Signed-off-by: Jason Volk <jason@zemos.net>

Author: jevolk

bindings/matrix-sdk-ffi/changelog.d/6606.added.md

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+Expose the [MSC3814] dehydrated-device manager on `Encryption`:
+`is_dehydrated_device_supported`, `create_dehydrated_device`,
+`rehydrate_dehydrated_device`, `delete_dehydrated_device`,
+`start_dehydrated_devices`, `stop_dehydrated_devices`, and a
+`dehydrated_device_event_listener` callback for lifecycle observability.
+
+[MSC3814]: https://github.com/matrix-org/matrix-spec-proposals/pull/3814

crates/matrix-sdk/changelog.d/6606.added.md

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+Add `Encryption::dehydrated_devices()`, a high-level [MSC3814] dehydrated-device
+manager that wraps the crypto-crate primitives (probe support, create,
+rehydrate, delete, weekly rotation, Secret Storage round trip for the pickle
+key, lifecycle event stream) and is modelled on `matrix-js-sdk`'s
+`DehydratedDeviceManager`. `MatrixMockServer` gains `mock_*` helpers for the
+four MSC3814 endpoints.
+
+[MSC3814]: https://github.com/matrix-org/matrix-spec-proposals/pull/3814