DTLS certificate fingerprint (a=fingerprint:) for SRTP keying.
What problem this solves
WebRTC ties DTLS-SRTP keys to the certificate fingerprint advertised in SDP.
Peers compare a=fingerprint: to the cert seen in the DTLS handshake before
sending media. This struct stores the hash algorithm and hex digest from SDP.
Peers compare the SDP fingerprint to the certificate presented during the DTLS handshake before deriving SRTP keys.
Wire form: a=fingerprint:<hash-algo> <fingerprint>