DictValidator: Allow non-string keys / customizable key validation

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Currently, the DictValidator only allows strings as dictionary keys.

Originally the reason for this was purely "we mostly want to use this with JSON and JSON doesn't support non-string keys anyway", but there are cases where non-string keys (e.g. integers) can be useful.

There are some open questions about this, though.

• It could(?) still make sense to have optional data type validation for keys, or even more specific/custom validation by specifying an arbitrary Validator as key_validator.
• If we allow key validators, it could be an option to have "light-weight" validation that only checks the type (so instead of specifying a Validator, just specify the type and the DictValidator will only do a isinstance check.
• Do we allow field validators for arbitrary key types too? (I don't see why not, but it could have some unexpected effects on testing, for example.)
• What should the default be? Allow any type, or keep the current behaviour (strings only) as the default?