REST API for retrieving OAuth access tokens

As preparation for an UI to retrieve OAuth tokens, a new endpoint
for the account REST API is added that returns a previously
obtained OAuth token:

GET /a/accounts/self/oauthtoken

The response will be 200 OK in case a token is available and the
response will contain a JSON body of the form

)]}'
{
  "username": "johndow",
  "resource_host": "git.example.org",
  "access_token": "eyJhbGciOiJSUzI1NiJ9.eyJqdGkiOi...",
  "providerId": "oauth-plugin:oauth-provider",
  "expires_at": "922337203775",
  "type": "bearer"
}

If there is no token available, or the token has already expired,
404 is returned. Attempts to retrieve a token of another user are
rejected with 403 Forbidden.

Change-Id: I6ddb825890e88c49bd8c5e66b8c5508cef7df347
Signed-off-by: Michael Ochmann <michael.ochmann@sap.com>

gerrit-server/src/main/java/com/google/gerrit/server/account/Module.java

@@ -64,6 +64,8 @@ public class Module extends RestApiModule {
     get(SSH_KEY_KIND).to(GetSshKey.class);
     delete(SSH_KEY_KIND).to(DeleteSshKey.class);
 
+    get(ACCOUNT_KIND, "oauthtoken").to(GetOAuthToken.class);
+
     get(ACCOUNT_KIND, "avatar").to(GetAvatar.class);
     get(ACCOUNT_KIND, "avatar.change.url").to(GetAvatarChangeUrl.class);