Add DTLS KeyLog configuration option in WebRTC API

Add an option in the setting engine to log TLS key material when a
DTLS connection is established with a peer.
The option exists in pion/dtls but is not easily accessible

settingengine.go

@@ -71,6 +71,7 @@ type SettingEngine struct {
 		clientAuth                *dtls.ClientAuthType
 		clientCAs                 *x509.CertPool
 		rootCAs                   *x509.CertPool
+		keyLogWriter              io.Writer
 	}
 	sctp struct {
 		maxReceiveBufferSize uint32
@@ -422,6 +423,12 @@ func (e *SettingEngine) SetDTLSRootCAs(rootCAs *x509.CertPool) {
 	e.dtls.rootCAs = rootCAs
 }
 
+// SetDTLSKeyLogWriter sets the destination of the TLS key material for debugging.
+// Logging key material compromises security and should only be use for debugging.
+func (e *SettingEngine) SetDTLSKeyLogWriter(writer io.Writer) {
+	e.dtls.keyLogWriter = writer
+}
+
 // SetSCTPMaxReceiveBufferSize sets the maximum receive buffer size.
 // Leave this 0 for the default maxReceiveBufferSize.
 func (e *SettingEngine) SetSCTPMaxReceiveBufferSize(maxReceiveBufferSize uint32) {