feat: Add context object to pass to supplier functions

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

@@ -1,9 +1,14 @@
 package com.google.auth.oauth2;
 
+import com.google.auth.oauth2.ExternalAccountCredentials.SubjectTokenTypes;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import java.io.Serializable;
 
-/** Context object to pass relevant variables from external account credentials to suppliers. */
+/**
+ * Context object to pass relevant variables from external account credentials to suppliers. This
+ * will be passed on any call made to {@link IdentityPoolSubjectTokenSupplier} or {@link
+ * AwsSecurityCredentialsSupplier}.
+ */
 public class ExternalAccountSupplierContext implements Serializable {
 
   private static final long serialVersionUID = -7852130853542313494L;
@@ -12,13 +17,13 @@ public class ExternalAccountSupplierContext implements Serializable {
   private final String subjectTokenType;
 
   /** Internal constructor. See {@link ExternalAccountSupplierContext.Builder}. */
-  ExternalAccountSupplierContext(Builder builder) {
+  private ExternalAccountSupplierContext(Builder builder) {
     this.audience = builder.audience;
     this.subjectTokenType = builder.subjectTokenType;
   }
 
   /**
-   * Gets the credentials expected audience.
+   * Returns the credentials' expected audience.
    *
    * @return the audience.
    */
@@ -27,7 +32,7 @@ public String getAudience() {
   }
 
   /**
-   * Gets the credentials expected subject token type.
+   * Returns the credentials' expected subject token type.
    *
    * @return the subject token type.
    */
@@ -45,8 +50,6 @@ public static class Builder {
     protected String audience;
     protected String subjectTokenType;
 
-    public Builder() {}
-
     /**
      * Sets the Audience.
      *
@@ -71,6 +74,18 @@ public Builder setSubjectTokenType(String subjectTokenType) {
       return this;
     }
 
+    /**
+     * Sets the subject token type.
+     *
+     * @param subjectTokenType the subjectTokenType to set.
+     * @return this {@code Builder} object
+     */
+    @CanIgnoreReturnValue
+    public Builder setSubjectTokenType(SubjectTokenTypes subjectTokenType) {
+      this.subjectTokenType = subjectTokenType.value;
+      return this;
+    }
+
     public ExternalAccountSupplierContext build() {
       return new ExternalAccountSupplierContext(this);
     }