Add a feature map to internal SIPCall. (#1615)

Author: dennwc

.changeset/sip-call-features.md

@@ -0,0 +1,5 @@
+---
+"github.com/livekit/protocol": patch
+---
+
+Add a feature map to internal SIPCall.

protobufs/rpc/io.proto

@@ -306,4 +306,5 @@ message SIPCall {
   repeated livekit.SIPUri via = 6;  // Via headers
   string sip_call_id = 7 [(logger.name) = "sipCallID"];           // SIP protocol generated call ID
   string project_id = 8 [(logger.name) = "projectID"];            // LiveKit project ID (Cloud only)
+  map<string,string> features = 9;
 }