add option to store package_filter into REZ_USED_PACKAGE_FILTER env-var

We have a case where our farm submitter uses some env-vars that rez sets in order to re-resolve the same env on the farm. Currently package filter is missing. That's because in theory a package filter could be very long (a long list of blacklisted pkgs for eg) and there is the chance of blowing some env-var char limit. Hence making this a configurable option, off by default.

Accompanying this we need a --package-filter option for rez-env that explicitly sets the package filter. Existing --exclude and --include filters have an additive effect on the default configured package filter, but in this case we need to replace it completely. This option would take a json-encoded string - the same string written to REZ_USED_PACKAGE_FILTER.