Add --ignore-unknown-fields that ignores unknown fields in rule group
files. There are lots of tools in the ecosystem that "like" to extend
the rule group file structure but they are currently unreadable by
promtool if there's anything extra. The purpose of this flag is so that
we could use the "vanilla" promtool instead of rolling our own.
Some examples of tools/code:
https://github.com/grafana/mimir/blob/main/pkg/mimirtool/rules/rwrulefmt/rulefmt.go8898eb3cc5/pkg/rules/rules.go (L18-L25)
Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@vinted.com>
