Bug 1612191 - Implement `mach browsertime --update-upstream-url $URL` for bumping version. r=Standard8,tarek

Might as well automate what we can automate.  I considered making the
"update package.json" logic part of setup_helper.py, so that it can be
used more broadly, but it's not clear it would be used with the
general push towards vendoring into the tree as part of `mach vendor`.

Depends on D63370

Differential Revision: https://phabricator.services.mozilla.com/D63371

--HG--
extra : moz-landing-system : lando