Add singletons for AwsCredentialsResolver impls (#1059)

Author: kstich

aws/client/aws-client-core/src/main/java/software/amazon/smithy/java/aws/client/core/identity/EnvironmentVariableIdentityResolver.java

@@ -25,6 +25,8 @@
  * </dl>
  */
 public final class EnvironmentVariableIdentityResolver implements AwsCredentialsResolver {
+    public static final EnvironmentVariableIdentityResolver INSTANCE = new EnvironmentVariableIdentityResolver();
+
     private static final String ACCESS_KEY_PROPERTY = "AWS_ACCESS_KEY_ID";
     private static final String SECRET_KEY_PROPERTY = "AWS_SECRET_ACCESS_KEY";
     private static final String SESSION_TOKEN_PROPERTY = "AWS_SESSION_TOKEN";

aws/client/aws-client-core/src/main/java/software/amazon/smithy/java/aws/client/core/identity/SystemPropertiesIdentityResolver.java

@@ -27,6 +27,8 @@
  * @see <a href="https://docs.oracle.com/javase/tutorial/essential/environment/sysprop.html">Java System Properties</a>
  */
 public final class SystemPropertiesIdentityResolver implements AwsCredentialsResolver {
+    public static final SystemPropertiesIdentityResolver INSTANCE = new SystemPropertiesIdentityResolver();
+
     private static final String ACCESS_KEY_PROPERTY = "aws.accessKeyId";
     private static final String SECRET_KEY_PROPERTY = "aws.secretAccessKey";
     private static final String SESSION_TOKEN_PROPERTY = "aws.sessionToken";

aws/client/aws-client-core/src/test/java/software/amazon/smithy/java/aws/client/core/identity/EnvironmentVariableIdentityResolverTest.java

@@ -15,7 +15,7 @@ public class EnvironmentVariableIdentityResolverTest {
 
     @Test
     void resolverReturnsExpectedIdentity() {
-        var resolver = new EnvironmentVariableIdentityResolver();
+        var resolver = EnvironmentVariableIdentityResolver.INSTANCE;
         var value = resolver.resolveIdentity(Context.empty());
         var expected = AwsCredentialsIdentity.create(
                 "env_access_key",

aws/client/aws-client-core/src/test/java/software/amazon/smithy/java/aws/client/core/identity/SystemPropertiesIdentityResolverTest.java

@@ -14,7 +14,7 @@
 public class SystemPropertiesIdentityResolverTest {
     @Test
     void resolverReturnsExpectedIdentity() {
-        var resolver = new SystemPropertiesIdentityResolver();
+        var resolver = SystemPropertiesIdentityResolver.INSTANCE;
         var value = resolver.resolveIdentity(Context.empty());
         var expected = AwsCredentialsIdentity.create(
                 "property_access_key",

cli/src/main/java/software/amazon/smithy/java/cli/SmithyCall.java

@@ -284,7 +284,7 @@ private void configureAuth(DynamicClient.Builder builder, ShapeId serviceInput,
                                 .profileName(auth.awsProfile);
                         builder.addIdentityResolver(new SdkCredentialsResolver(profileBuilder.build()));
                     } else {
-                        builder.addIdentityResolver(new EnvironmentVariableIdentityResolver());
+                        builder.addIdentityResolver(EnvironmentVariableIdentityResolver.INSTANCE);
                     }
                     break;
                 case "none":