multi: add notifications API

multi: add notifications API

Add infrastructure to colled and dispatch notifications for transfers
and the multi handle in general. Applications can register a callback
and en-/disable notification type the are interested in.

Without a callback installed, notifications are not collected. Same when
a notification type has not been enabled.

Memory allocation failures on adding notifications lead to a general
multi failure state and result in CURLM_OUT_OF_MEMORY returned from
curl_multi_perform() and curl_multi_socket*() invocations.

Closes #18432