Modernise the way proxy connections work

[brendanheywood]

This will redo the work done in this issue:

#9

and do it so that all the proxy logic is handled centrally in local_aws instead

https://github.com/catalyst/moodle-local_aws/blob/master/classes/local/client_factory.php#L44-L60

[Peterburnett]

The strategy here will be to add a middleware to the guzzle stack, which configures the proxy settings for the request, based on the URL of the request, as in the above function.

The main function is currently protected, and the best strategy here will likely be to copy the Awsclient implementation to a new function that accepts a GuzzleClient, and from there, configures the middleware into the callstack in the same way. The same middleware function should work but might need some tweaking