feat: increase ImdsClient.Builder.engine visibility to public

Author: ianbotsf

.changes/3bc2e0a5-d8d4-4051-a723-a54427a96e7e.json

@@ -0,0 +1,5 @@
+{
+    "id": "3bc2e0a5-d8d4-4051-a723-a54427a96e7e",
+    "type": "feature",
+    "description": "Enable configuring an HTTP engine on `ImdsClient`"
+}

aws-runtime/aws-config/common/src/aws/sdk/kotlin/runtime/config/imds/ImdsClient.kt

@@ -172,9 +172,9 @@ public class ImdsClient private constructor(builder: Builder) : InstanceMetadata
         public var logMode: LogMode = LogMode.Default
 
         /**
-         * The HTTP engine to use to make requests with. This is here to facilitate testing and can otherwise be ignored
+         * The HTTP engine to use to make requests with
          */
-        internal var engine: HttpClientEngine? = null
+        public var engine: HttpClientEngine? = null
 
         /**
          * The source of time for token refreshes. This is here to facilitate testing and can otherwise be ignored