Added support for SRTP_NULL_HMAC_SHA1_80 cipher

Added support for SRTP_NULL_HMAC_SHA1_80 protection profile (cipher).
It is disabled by default. You need to use SettingEngine and set list
of allowed SRTP protection profiles using its SetSRTPProtectionProfiles
function called with dtls.SRTP_NULL_HMAC_SHA1_80 as a parameter. You
need to do this for both pion peers. For non-pion ones you may need to
enable it somewhere too, as NULL cipher is usually disabled for security
reasons.

dtlstransport.go

@@ -374,6 +374,8 @@ func (t *DTLSTransport) Start(remoteParameters DTLSParameters) error {
 		t.srtpProtectionProfile = srtp.ProtectionProfileAeadAes256Gcm
 	case dtls.SRTP_AES128_CM_HMAC_SHA1_80:
 		t.srtpProtectionProfile = srtp.ProtectionProfileAes128CmHmacSha1_80
+	case dtls.SRTP_NULL_HMAC_SHA1_80:
+		t.srtpProtectionProfile = srtp.ProtectionProfileNullHmacSha1_80
 	default:
 		t.onStateChange(DTLSTransportStateFailed)
 		return ErrNoSRTPProtectionProfile