floss: Add signals to hcidoc

Feedback analyzer wants specific tags to bucket issues. To support this,
we add the concept of signals, which contain a packet index, timestamp
and a tag. Emit the Nocp signal as the first implementation of this.

Bug: 262928525
Tag: #floss
Test: ./build.py --target utils (and run binary)
Change-Id: I964919e70fc13fa1472790309a9dd4f8517cf651