feat(grouping): Add replacement callback option to parameterizer

[lobsterkatie]

This adds the ability to specify a replacement callback when defining a regex for the parameterizer. For example, a regex can now be defined like this:

ParameterizationRegex(
    name="dog",
    raw_pattern=r"some pattern",
    replacement_callback=lambda orig_value: "<dog>" if some_condition else orig_value,
)

When the parameterizer finds a matching value, instead of just reflexively replacing it with the regex's name, it'll pass the original value to the callback, and let it decide what the replacement value will be. (This is going to be necessary for us to fix our IPv6 parameterization.)