Moves main into cmd dir. (#977)

cmd/fnserver/main.go

@@ -0,0 +1,14 @@
+package main
+
+import (
+	"context"
+
+	"github.com/fnproject/fn/api/server"
+	// EXTENSIONS: Add extension imports here or use `fn build-server`. Learn more: https://github.com/fnproject/fn/blob/master/docs/operating/extending.md
+)
+
+func main() {
+	ctx := context.Background()
+	funcServer := server.NewFromEnv(ctx)
+	funcServer.Start(ctx)
+}