Developer convenience: allow curl to ignore ssl issues

When working on a local dev box which is ssl but doesn't have a proper cert it would be useful to have a simple option to allow curl to work anyway.

I was originally thinking a single CFG toggle which changes these:

https://github.com/moodle/moodle/blob/master/lib/filelib.php#L3264-L3265

But then I thought it might be more useful and flexible to have something like:

$CFG->curldefaults = [

    'CURLOPT_SSL_VERIFYPEER' => 0,

    'CURLOPT_SSL_VERIFYHOST' => 2,

    'CURLOPT_CONNECTTIMEOUT' => 10,

];

In particular some of the other settings like the various timeouts may be useful to override in production too.