Add spec change for updating capabilities after initial message (#217)

Author: jaronoff97

specification.md

@@ -560,6 +560,12 @@ in the future such that old Agents automatically report that they don't
 support the new capability.
 This field MUST be always set.
 
+An Agent MAY update any of its capabilities at any time after the first
+message. The server MUST respect the Agent's changes, ensuring that the
+subsequent messages are sent correctly. If the server sends a part of its
+messages corresponding to a capability the Agent doesn't support, the Agent
+SHOULD ignore it.
+
 ```protobuf
 enum AgentCapabilities {
     // The capabilities field is unspecified.