Napi search path (#490)

Co-authored-by: Bret Ambrose <bambrose@amazon.com>

Author: bretambrose

lib/native/binding.js

@@ -58,13 +58,27 @@ if (existsSync(dist)) {
 
 const bin_path = path.resolve(source_root, 'bin');
 
-const search_paths = [
-    path.join(bin_path, platformDir, binary_name),
+let search_paths = [
+    path.join(bin_path, platformDir, binary_name) + '.node',
 ];
 
+/*
+ * Environment variables can inject (at lower-priority) paths into the search process as well.  Support both relative
+ * and absolute path overrides.
+ */
+let relative_path = process.env.AWS_CRT_NODEJS_BINARY_RELATIVE_PATH;
+if (relative_path) {
+    let final_path = path.resolve(__dirname, ...relative_path.split(path.sep))
+    search_paths.push(final_path);
+}
+
+if (process.env.AWS_CRT_NODEJS_BINARY_ABSOLUTE_PATH) {
+    search_paths.push(process.env.AWS_CRT_NODEJS_BINARY_ABSOLUTE_PATH);
+}
+
 let binding;
 for (const path of search_paths) {
-    if (existsSync(path + '.node')) {
+    if (existsSync(path)) {
         binding = require(path);
         break;
     }