Optional inner validators for `deep_mapping`

[AdrianSosic]

This is a copy of the fourth issue collected in https://github.com/python-attrs/attrs/issues/1206 to enable separate tracking of the bug.

The issue

For deep_iterable, only the iterable_validator is optional, which makes sense because if no member_validator was provided, one should simply use a regular validator in the first place. However, for deep_mapping, both value_validator and key_validator are required. This makes its use unnecessarily complicated in certain situations. For example, validating only dictionary keys requires a rather cumbersome workaround and also results in unnecessary calls of the value_validator:

@define
class B:
    x: Dict[str, Any] = field(
        validator=deep_mapping(
            key_validator=and_(instance_of(str), min_len(1)),
            value_validator=lambda *x: None,
        )
    )

Instead, I guess the user-friendly way would be to check that at least one of them is provided, but not necessarily both.