[new feature] Support for filtering sandboxes by labels

[foxish]

New feature or functionality

What would you like Signadot to do?

This functionality would allow users to fetch only a subset of the sandboxes that they're interested in based on the presence of a certain label name / value association.

Additional considerations

Anything in particular to think about regarding this new feature?

We want to additionally allow this filtering to happen within the browser extension as well.