Validator All Unique - Tune Up to make error explicit

[DEKHTIARJonathan]

This PR replaces the following:

lambda v: validate_list_all_unique(
    v, key=attrgetter("feature_hash")
)

By the following:

lambda v: validate_list_all_unique(
    v, keys=("namespace", "feature")
)

The new validator is preferable (though equivalent) to avoid a cryptic error message:

# Before
variantlib.errors.ValidationError: Duplicate value found: '7153903334020990830' in list.

# After
variantlib.errors.ValidationError: Duplicate value found: '('my_namespace', 'my_feature')' in list.

[mgorny]

LGTM