Allow overriding the base URL in the javascript Navigator SDK (#445)

Author: stefan-kickoff

docs/navigator_api/js/streaming_navigator.js

@@ -8,7 +8,7 @@ const DEFAULT_HYPERPARAMS = {
   top_p: 0.9,
 };
 
-const BASE_URL = "https://api.gretel.cloud/";
+const BASE_URL = process.env.GRETEL_BASE_URL || "https://api.gretel.cloud/";
 
 ///////////////////////////////////////////////
 // Generates structured data given a prompt //