Add SlackTokens option for slack credentials (#647)

proto/credentials.proto

@@ -59,3 +59,8 @@ message GitHubApp {
   string installation_id = 2;
   string app_id = 3;
 }
+
+message SlackTokens {
+  string app_token = 1;
+  string bot_token = 2;
+}

proto/sources.proto

@@ -189,6 +189,7 @@ message Slack {
   string endpoint = 1 [(validate.rules).string.uri_ref = true];
   oneof credential {
     string token = 2;
+    credentials.SlackTokens tokens = 5;
   }
   repeated string channels = 3;
   repeated string ignoreList = 4;