Add support to the CHT api for new endpoint for OIDC login

[jkuester]

• The /medic/login/oidc endpoint should support GET requests with a code query param. The code should be the authorization_code coming from the OIDC Provider.
• This is the endpoint the OIDC Provider should re-direct back to.
• Add a new getOidc function in api/src/controllers/login.js that will handle the request and call through to code in the openid-client to validate the authorization_code.
• For a good authorization_code, the OIDC Provider should return an id_token containing the username for the authenticated user.
• Lookup the user's doc in _userand confirm oidc_provider value is set
• Generate a Couch session cookie for user and respond with valid user session.
  • This will require access to the Couch secret value which can be requested from the Couch instance. There is some code in the proxy auth PR that does this. I think we might be able to put that functionality for getting the secret into shared-libs/settings (or maybe that code is already sufficient for retrieving the secret value?).
• Add integration tests for the login controller

[benkags]

@jkuester please assign me this ticket

[jkuester]

@benkags Done!