It's required by the handler but not getting passed to Axum's middleware
stack.

As the client doesn't import "big" corro-types, move public facing
types to the dedicated crate.

The client is much simpler than the existing subscription client:
 - it doesn't handle I/O error and simply bubble it up (as the user
   still need to handle it)
 - the only possible errors are I/O and deserialization as the /updates
   stream doesn't have a "change id" concept