feat: support cohere (close #195)

relay/channel/cohere/constant.go

@@ -0,0 +1,7 @@
+package cohere
+
+var ModelList = []string{
+	"command-r", "command-r-plus", "command-light", "command-light-nightly", "command", "command-nightly",
+}
+
+var ChannelName = "choere"