[iOS] Support for ISI JWT

[arturogutierrez]

MON-1577

Adding support for the new ISI JWT. At this moment, we should only store it and resend it in further calls.

How is it implemented?

I tried to follow the RC SDK codebase. Basically:

• I added a new JWTManager responsible for storing/retrieving the token from the cache
• HTTPClient forwards the URLResponse to the JWTManager to store the cookie if present
• HTTPClient adds the cookie to all requests if present
• Created a new public API for the RC SDK to get the current token or refresh it if it's expired

[nesterenkodm]

lgtm