Enable sniffing of HCI packets to std::ostream

This allows for the registration of an ostream on the hci sniffer. This
can be used by external parties to sniff hci packets and write them to
different streams.

Bug: 231598376
Bug: 199325666
Tag: #refactor
Test: Two emulators can discover each other
Test: gd/cert/run
Change-Id: I0622263eaec8dd360d893817dd6a73f30a15af5f