Can/should marshmallow include pre-built validates_schema methods?

I have a case in which I'm looking at adding a mutually exclusive set of fields to a schema.
It would be awesome if I could define this behavior with `marshmallow.validate.MutuallyExclusive(["foo", "bar"])`, and I'd be happy to work on an implementation of that. But where would it go and how would users apply it?

Unless I've missed/forgotten something, there's no way to provide pre-packaged schema validators for users to apply.

The following could work but is ugly:
```python
class MySchema(Schema):
foo = fields.String()
bar = fields.String()

_foo_bar_mutex = validates_schema(MutuallyExclusive(["foo", "bar"]))
```

What about a class decorator which adds a validator?

```python
@with_validates_schema(MutuallyExclusive(["foo", "bar"]))
class MySchema(Schema):
foo = fields.String()
bar = fields.String()
```

Is this worth pursuing? And if so, what is the correct interface to build?

Start by reviewing the existing marshmallow.validate and validates_schema APIs, along with the Schema usage shown in the issue. Determine whether pre-built schema validators should be supported and what interface best applies MutuallyExclusive-style behavior. Done means an agreed public interface and implementation scope, with its application to the mutually exclusive foo and bar fields made clear.

Written by the indexing model from the issue text.