fix: managed upload resolves config repeatedly (#3700)

* fix: managed upload not to resolve config repeatedly

Previously the managed upload creates new S3 clients with the
constructor parameters of the S3 client used to create the managed
upload. That results in that the newly created S3 client needs to
resolve every config, including the credentials over and over
again.

This issue was introduced by #3109 to make sure the internal S3
client inside of managed upload has the same constructor parameters
of the S3 client used to create the managed upload. So that the
logic relies on the constructor parameters like access point
and endpoint discovery works properly.

This change supply the resolved client config to create the
managed upload internal S3 client so configs don't need to be
resolved again, like credentials. Meanwhile, we explicitly set
the constructor parameter to be the same as that of S3 client
used to create the managed upload.

* address feedbacks

Co-authored-by: Trivikram Kamat <16024985+trivikr@users.noreply.github.com>

.changes/next-release/bugfix-ManagedUpload-2bd31158.json

@@ -0,0 +1,5 @@
+{
+  "type": "bugfix",
+  "category": "ManagedUpload",
+  "description": "Use resolved credentials if customer supplies configured S3 Client"
+}

lib/s3/managed_upload.js

@@ -283,12 +283,18 @@ AWS.S3.ManagedUpload = AWS.util.inherit({
     if (!self.service) {
       self.service = new AWS.S3({params: params});
     } else {
+      // Create a new S3 client from the supplied client's constructor.
       var service = self.service;
-      var config = AWS.util.copy(service._originalConfig || {});
+      var config = AWS.util.copy(service.config);
       config.signatureVersion = service.getSignatureVersion();
       self.service = new service.constructor.__super__(config);
       self.service.config.params =
         AWS.util.merge(self.service.config.params || {}, params);
+      Object.defineProperty(self.service, '_originalConfig', {
+        get: function() { return service._originalConfig; },
+        enumerable: false,
+        configurable: true
+      });
     }
   },