Add support for comma-separated groups in group mapping attribute

[julien-nc]

Some IdPs can't provide JSON values in token attributes.

[janepie]

Looks good :)

[edward-ly]

In case there is any excess whitespace in the string, maybe it would also be a good idea to use trim/mb_trim to remove them.
wdyt?

[julien-nc]

Sure, it's now:

if (is_string($groups)) {
	$groups = explode(',', $groups);
	// remove surrounding spaces in each group
	$groups = array_map('trim', $groups);
	// remove empty strings
	$groups = array_filter($groups);
}