Add support for variable account data type names

[Frinksy]

MSC1946 introduced new events types. It is also now part of the unstable spec, here.
One of the new events is of type m.secret_storage.[key_ID] where [key_ID] is to be substituted by the ID of the key to store.
This means that there could be an arbitrary number of event type names to store keys, e.g. m.secret_storage.key_1, m.secret_storage.key_2, etc.

Also, it seems that secrets will be stored with entirely variable event type names (from here):

Encrypted data is stored in the user’s account_data using the event type defined by the feature that uses the data.

Currently, Ruma's macros only support event types with a fixed name. To support these new events with variable type names, ruma-events-macros needs to be refactored to allow this.

Although secret sharing is already being implemented in (#677), the secret storage part of the implementation is blocked, meaning #478 depends on this issue being resolved.

[jplatte]

Actually, this is only needed for account data "events" that aren't really events in the first place. I will tackle this in conjunction with #732.

[jplatte]

Note to self: EventType::Foo for a variable type will have to be EventType::Foo(String), which also means we'll need more derive code...

And this actually kind-of clashes with some earlier thoughts by myself to have the type be a DST wrapping str with associated constants. That could still work, but the nice matching wouldn't work that way, instead there would have to be some as_foo(&self) -> Option<&str> helpers.