Allow duplicate keys across attribute/type filters

[tcompa]

Observed:

I don't think these keys have to be unique across the two kinds of filters, but I will double check.

[tcompa]

I double-checked in

• WorkflowTaskUpdateV2 schema: https://github.com/fractal-analytics-platform/fractal-server/blob/5d8a2f097dfa4cacd24b3bf18b001008e0922f17/fractal_server/app/schemas/v2/workflowtask.py#L108-L114
• Filters schema: https://github.com/fractal-analytics-platform/fractal-server/blob/main/fractal_server/images/models.py#L113-L134

and found no reason for rejecting duplicate keys in the two different parts (attributes and types)