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
4578763c7a