CORS support should be optional

[uniqueg]

While I don't understand tne implications fully, I think [CORS][] should not be enabled for every microservice and thus should be made optional.

It's rather an easy fix but needs some changes all around (code, docstrings, possibly examples).

In principle, this is what needs to be done, as far as I can tell:

• A config parameter enable_cors should be added to the SecurityConfig model (set default to True) in foca/models/config.py; update docstring accordingly, including example and, if necessary, dependent examples (search file for any mentions of SecurityConfig)
• In the main FOCA file (foca/foca.py), the value of the config parameter should be read to decide whether to call enable_cors() in line 49 (i.e., add an if ... around that call and the log message, maybe add another log message to say that CORS is disabled in the else ... block)
• Check if any documentation needs to be updated

[vipulchhabra99]

Can you please assign me this issue? @uniqueg