Implement Interceptors

Provide API so that handling around RTP can be easily defined by the
user. See the design doc here[0]

[0] https://github.com/pion/webrtc-v3-design/issues/34

api.go

@@ -3,6 +3,7 @@
 package webrtc
 
 import (
+	"github.com/pion/interceptor"
 	"github.com/pion/logging"
 )
 
@@ -13,6 +14,7 @@ import (
 type API struct {
 	settingEngine *SettingEngine
 	mediaEngine   *MediaEngine
+	interceptor   interceptor.Interceptor
 }
 
 // NewAPI Creates a new API object for keeping semi-global settings to WebRTC objects
@@ -35,6 +37,10 @@ func NewAPI(options ...func(*API)) *API {
 		a.mediaEngine = &MediaEngine{}
 	}
 
+	if a.interceptor == nil {
+		a.interceptor = &interceptor.NoOp{}
+	}
+
 	return a
 }
 
@@ -57,3 +63,11 @@ func WithSettingEngine(s SettingEngine) func(a *API) {
 		a.settingEngine = &s
 	}
 }
+
+// WithInterceptorRegistry allows providing Interceptors to the API.
+// Settings should not be changed after passing the registry to an API.
+func WithInterceptorRegistry(interceptorRegistry *interceptor.Registry) func(a *API) {
+	return func(a *API) {
+		a.interceptor = interceptorRegistry.Build()
+	}
+}