bugzilla: add a rest api backend (usable with bugzilla 5.0+) Add support for the bugzilla rest api documented at https://wiki.mozilla.org/Bugzilla:REST_API and at https://bugzilla.readthedocs.io/en/latest/ This backend has the following benefits: * It supports the bugzilla api keys so hgrc does not need to contain a user's bugzilla password * Works with Mercurial's "hostfingerprints" support making handling bugzilla instances with self-signed certs easier * Does not use xmlrpc ;-) Adds configuration item 'apikey' in [bugzilla] section. My major concern with these patches is if the approach to HTTP access is the right way for an extension and if hooking into request object and the overriding the get_method to perform PUT requests was a sensible approach. # no-check-commit