Fixed bug where using profile-based credentials could cause the SDK to read the profile file with each request. (#3950)

This bug was introduced with the profile file supplier changes. If the customer was not overriding the profile file via the client override config AND was relying on profile file-based configuration, that configuration would be read with every request. This had a significant performance cost.

```
Benchmarking Results (software.amazon.awssdk.benchmark.apicall.protocol.JsonProtocolBenchmark)
Before:   2858.805 ± 181.491  ops/s
After:    10915.079 ± 677.022  ops/s
```

Author: millems

.changes/next-release/bugfix-AWSSDKforJavav2-c9734ed.json

@@ -0,0 +1,6 @@
+{
+    "category": "AWS SDK for Java v2", 
+    "contributor": "", 
+    "type": "bugfix", 
+    "description": "Fixed bug where using profile-based credentials could cause the SDK to read the profile file with each request."
+}

core/sdk-core/src/main/java/software/amazon/awssdk/core/client/builder/SdkDefaultClientBuilder.java

@@ -264,9 +264,9 @@ protected SdkClientConfiguration mergeChildDefaults(SdkClientConfiguration confi
      */
     private SdkClientConfiguration mergeGlobalDefaults(SdkClientConfiguration configuration) {
         // Don't load the default profile file if the customer already gave us one.
-        Supplier<ProfileFile> configuredProfileFileSupplier = configuration.option(PROFILE_FILE_SUPPLIER);
-        Supplier<ProfileFile> profileFileSupplier = Optional.ofNullable(configuredProfileFileSupplier)
-                                                            .orElseGet(() -> ProfileFile::defaultProfileFile);
+        Supplier<ProfileFile> profileFileSupplier =
+            Optional.ofNullable(configuration.option(PROFILE_FILE_SUPPLIER))
+                    .orElseGet(() -> ProfileFileSupplier.fixedProfileFile(ProfileFile.defaultProfileFile()));
 
         return configuration.merge(c -> c.option(EXECUTION_INTERCEPTORS, new ArrayList<>())
                                          .option(ADDITIONAL_HTTP_HEADERS, new LinkedHashMap<>())

core/sdk-core/src/test/java/software/amazon/awssdk/core/client/builder/DefaultClientBuilderTest.java

@@ -52,6 +52,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
+import java.util.function.Supplier;
 import org.assertj.core.api.Assertions;
 import org.junit.Before;
 import org.junit.Test;
@@ -371,7 +372,19 @@ public void clientBuilderFieldsHaveBeanEquivalents() throws Exception {
                 assertThat(property.getWriteMethod()).as(propertyName + " setter").isNotNull();
             });
         });
+    }
+
+
+    @Test
+    public void defaultProfileFileSupplier_isStaticOrHasIdentityCaching() {
+        SdkClientConfiguration config =
+            testClientBuilder().build().clientConfiguration;
+
+        Supplier<ProfileFile> defaultProfileFileSupplier = config.option(PROFILE_FILE_SUPPLIER);
+        ProfileFile firstGet = defaultProfileFileSupplier.get();
+        ProfileFile secondGet = defaultProfileFileSupplier.get();
 
+        assertThat(secondGet).isSameAs(firstGet);
     }
 
     private SdkDefaultClientBuilder<TestClientBuilder, TestClient> testClientBuilder() {