feat: add workload identity federation support (#547)

Adds new credential types for File/URL based external credentials.
Adds utilities for STS token exchange

See go/guac-3pi-java.

Co-authored-by: Jeff Ching <chingor@google.com>

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

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2021 Google LLC
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *    * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *    * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *
+ *    * Neither the name of Google LLC nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package com.google.auth.oauth2;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+
+/**
+ * The acting party as defined in <a href="https://tools.ietf.org/html/rfc8693">OAuth 2.0 Token
+ * Exchange</a>.
+ */
+final class ActingParty {
+  private final String actorToken;
+  private final String actorTokenType;
+
+  ActingParty(String actorToken, String actorTokenType) {
+    this.actorToken = checkNotNull(actorToken);
+    this.actorTokenType = checkNotNull(actorTokenType);
+  }
+
+  String getActorToken() {
+    return actorToken;
+  }
+
+  String getActorTokenType() {
+    return actorTokenType;
+  }
+}