Suggestion: Refactor builtin fields to expose validators for re-use

Nobody has claimed this yet.

For my use case, I want to have a custom field-getter using `fields.Function()` but the value it gets will be a UUID. I'd like to be able to have the same validation as the builtin `fields.UUID()`. However, that doesn't seem to be [available in the `validate` module.](https://github.com/marshmallow-code/marshmallow/blob/dev/marshmallow/validate.py)

My suggestion is to refactor all the builtin field types so that they call validators in `validate`. That way, the same validators would also be exposed for reuse in custom user-defined fields.

As I'm imagining this, it would be a fully backwards-compatible change and the only difference to the public interface would be documenting/exposing new validators.

If that sounds like a good idea to the maintainers, I'm happy to take a crack at this and open a PR.

Start with marshmallow/validate.py and the implementations of fields.Function() and fields.UUID(), then compare how builtin field types currently perform validation. Define the reusable-validator scope and check that builtin behavior remains unchanged while validators are exposed for custom fields; confirm the public interface and documentation reflect the new exports.

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