Add REST API endpoints for user's contribution agreements

Add two new REST API endpoints to list the user's contribution
agreements, and to enter a contribution agreement.

The list of contribution agreements differs from the existing RPC
endpoint in that it only returns a list of the agreements that the
user has entered; it does not include a list of existing agreements.

Bug: Issue 4316
Change-Id: I7d988aa59c06380767d888e22f6e0eaad62d8b34

gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/AbstractDaemonTest.java

@@ -309,6 +309,10 @@ public abstract class AbstractDaemonTest {
     return cfg.getBoolean("change", null, "submitWholeTopic", false);
   }
 
+  protected boolean isContributorAgreementsEnabled() {
+    return cfg.getBoolean("auth", null, "contributorAgreements", false);
+  }
+
   protected void beforeTest(Description description) throws Exception {
     GerritServer.Description classDesc =
       GerritServer.Description.forTestClass(description, configName);