This allows deserializing all the `*EventContent` types into a parent `Any{...}EventContent`, assuming we know the type of the underlying event.
Required for serializing/deserializing the content of events we'd like to send, across application restarts, as in https://github.com/matrix-org/matrix-rust-sdk/issues/3361 for the Rust SDK.
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* events: add deserialize_with_type to all the *EventContent types
* events: add smoke test for deserializing an event content with a type
* events: add a test for deserializing a secret storage key event content
* events: add fix for correctly matching events with a type fragment
* Address review comments.
