[Macros] extract value of a client-write

[hexpwn]

Currently is macros it is possible to wait-for-write; but other than asserting the value that was received, we cannot use that value elsewhere.

One example would be sending a notification that incorporates that value (maybe for a request-response situation, for example).

It would be nice if we could save the received value to a variable of sorts and use it elsewhere in other operations.