Typically a stack needs to add the callback for a sock as a member of
its respective `sock` type so `sock_types.h` needs to include
`net/sock/async.h` at the moment. As those however include
`net/sock/<prot>.h`, which in turn include `sock_types.h`, we create a
cyclic dependency.
This fix resolves this cyclic dependency, by putting the callback
definitions in its own header that then in turn can be also included
by `sock_types.h`.
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