Creates a new access token for use in Cyan Web API request authentication/authorization.
The POST body is empty, and there are no URL/query string args, but use a POST here because
the server is creating something and we want to maintain good REST semantics
(no caching for POST/idempotency/etc.)
The resulting access token can be used as an alternative to HTTP Basic Authentication for
accessing the Cyan Web API.
For HTTP Basic authentication, the Web API caller normally adds a request header like this:
Authorization: Basic {base64 encoded username:password}
To use the Cyan access token authentication, you must first request a new token by invoking
this API (using HTTP Basic Authentication credentials). Once you have obtained the token, you can
substitute the HTTP Basic Authentication header in successive requests with the following:
Authorization: CyanAccessToken {base64 encoded token, provided by this API}
Access tokens are set to expire after a fixed interval, and do not automatically renew or
"slide" the expiration time with each use. After a token has expired, the caller must
request a new token using this API.