Support generic emitted deprecation messages

IOW -- it should be possible to do something like `Deprecator.emit(message, **policy_args)`, which emits a generic `Kind` of deprecation.

For sample use, see e.g. [here](https://github.com/Julian/jsonschema/blob/b0be7bf48e6554a00296307f7c84283a77738267/jsonschema/validators.py#L948-L953).

It likely should support similar policy arguments (e.g. removal date).

Start by reviewing the existing Deprecator API and the linked jsonschema/validators.py example. Define what generic Deprecator.emit(message, **policy_args) should accept, including policy arguments such as a removal date, and consider the work complete when it emits a generic deprecation Kind with those policies applied.

Written by the indexing model from the issue text.