feat: add extra-filter-labels param

[Goorsky1]

Motivation and use cases

This param is mostly useful when using Sloth with a metric storage for multiple k8s clusters (like VictoriaMetrics).

Even if you would define a unique name for each SLO and provide a proper filter in each query, some of the generated rules would end up without the required filter and you would result in an error when executing the generated rules: result contains metrics with the same labelset after applying rule labels

Assuming you define --extra-filter-labels cluster=my-cluster-name for Sloth operator,
It allows for using the same SLO names on each cluster as {{ .cluster }} can be referenced in the user's query and also will be used as filter in the created recording rules, which users can't modify, fixing previously described problem. The cluster label will be also added to the resulting metrics in similar fasion to --extra-labels, so it ensures that the resulting metrics can be differentiated.

This fixes issues like #430 or can be helpful for #349

[zmailloux]

Wish this would just be pulled in.

[nirmalkumar85]

+1 bump. Thanks.