Add PEP 712 converters to the `dataclasses` spec

[thejcannon]

Picking up from https://discuss.python.org/t/would-adding-converter-to-dataclass-transform-even-need-a-pep/55125

Which stems from https://discuss.python.org/t/pep-712-adding-a-converter-parameter-to-dataclasses-field/26126 (already discussed) this PR adds converter as another field specifier parameter.