Add new security review API client

[DanielRyanSmith]

This is part of the work for integrating ChromeStatus with IssueTracker API in order to create and reference security review bugs. A number of requests to the OT API will need to be implemented as a ChromeStatus client.

Two new fields should be added to the FeatureEntry kind:
continuity_id: integer
security_bug_id: integer

Example for adding a new field (this example is adding a field to the Stage kind, however).

This will require creating a new file similar to origin_trials_api.py.

These actions will need to be handled:

• Check if a given continuity ID is valid, and to check if a launch bug exists for that continuity ID [GET, do_get()].
• Create a new launch bug [POST, do_post()].

For each of these actions, a new function should be added to origin_trials_client.py that will be used by the new API endpoints.