Allow specifying extra websockets handler directories
This allows vendors to write their own websockets handlers for
non-shared tests.
Note that the handlers still all share the same namespace, so vendor
handlers must have a globally unique name, not just unique in the
vendor directory.
Differential Revision: https://phabricator.services.mozilla.com/D222758
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1919741
gecko-commit: 91b6ceb89eae8391c7b913f52e600e3bb04c412e
gecko-reviewers: Sasha