Implement api request logging to Redis Streams.

This makes it possible for external services to subscribe (through
Redis) for realtime events. E.g. a create, update or delete device event
could trigger an external synchronization.

api/python/proto/chirpstack-api/api/request_log.proto

@@ -0,0 +1,23 @@
+syntax = "proto3";
+
+package api;
+
+option go_package = "github.com/chirpstack/chirpstack/api/go/v4/api";
+option java_package = "io.chirpstack.api";
+option java_multiple_files = true;
+option java_outer_classname = "RequestLog";
+
+import "google/protobuf/timestamp.proto";
+import "chirpstack-api/common/common.proto";
+import "chirpstack-api/gw/gw.proto";
+
+message RequestLog {
+    // API service name.
+    string service = 1;
+
+    // API method name.
+    string method = 2;
+
+    // Metadata.
+    map<string, string> metadata = 3;
+}