[API][v1] Add a better-described wrapper type for EventKey

[banool]

Currently with the API, e.g. /events/:event_key, the key manifests as just a string in the spec (though the format field is set, which is a start). Ideally it would instead appear as a wrapper type, perhaps with some kind of spec-side "validation", e.g. using the pattern field to make it clear that it's meant to be a number + address as hex.

We should do something similar with every type that uses impl_poem_parameter.