Support attribute specifications for annotations

[zhiltsov-max]

Currently, all annotations can have arbitrary attributes, but there is no specifications for these attributes. It would be great, if Datumaro provided:

• A list of possible attribute values (at least, existing in a dataset)
• A type for each attribute (categorical or contiguous/numerical)
• A data type for each attribute

Most dataset formats do not include such information, however. As the first step, we could:

• consider all attributes categorical. Try to recognize, if an attribute is numerical
• infer value ranges from values in input data