fix: Override toBuilder() for ExternalAccountCredential and subclasses (#1793)

* fix: Override toBuilder() for ExternalAccountCredential and subclasses

* chore: Update to use local Builder class

* chore: Set a default impl for toBuilder in ExternalAccountCredentials

* chore: Remove override from ExternalAccountCredentials

Author: lqiu96

oauth2_http/java/com/google/auth/oauth2/AwsCredentials.java

@@ -233,6 +233,11 @@ public static AwsCredentials.Builder newBuilder(AwsCredentials awsCredentials) {
     return new AwsCredentials.Builder(awsCredentials);
   }
 
+  @Override
+  public Builder toBuilder() {
+    return new Builder(this);
+  }
+
   public static class Builder extends ExternalAccountCredentials.Builder {
 
     private AwsSecurityCredentialsSupplier awsSecurityCredentialsSupplier;

oauth2_http/java/com/google/auth/oauth2/IdentityPoolCredentials.java

@@ -153,6 +153,11 @@ public static Builder newBuilder(IdentityPoolCredentials identityPoolCredentials
     return new Builder(identityPoolCredentials);
   }
 
+  @Override
+  public Builder toBuilder() {
+    return new Builder(this);
+  }
+
   private IdentityPoolSubjectTokenSupplier createCertificateSubjectTokenSupplier(
       Builder builder, IdentityPoolCredentialSource credentialSource) throws IOException {
     // Configure the mTLS transport with the x509 keystore.

oauth2_http/java/com/google/auth/oauth2/PluggableAuthCredentials.java

@@ -203,6 +203,11 @@ public static Builder newBuilder(PluggableAuthCredentials pluggableAuthCredentia
     return new Builder(pluggableAuthCredentials);
   }
 
+  @Override
+  public Builder toBuilder() {
+    return new Builder(this);
+  }
+
   @VisibleForTesting
   @Nullable
   ExecutableHandler getExecutableHandler() {