It seems that we cant count on serde_json::to_value to detect
duplicates,
because it swallows them instead of returning an error.
The only solution then is to serialize to string and try to deserialize
again.
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.
---
* 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.
