Allow passing a Field instance to unknown option

[Br1an67]

Summary

Allow a Field instance to be passed as the unknown option (in addition to the existing RAISE, INCLUDE, EXCLUDE constants). When a Field is provided, unknown field values are deserialized through that field, enabling validation and type coercion.

Closes #853

Example

from marshmallow import Schema, fields

class MySchema(Schema):
    name = fields.String()

schema = MySchema(unknown=fields.Int())
schema.load({"name": "Joe", "age": "42"})
# => {"name": "Joe", "age": 42}

The Field can be set via:

• Schema(unknown=fields.Int())
• class Meta: unknown = fields.Int()
• schema.load(data, unknown=fields.Int())
• fields.Nested(ChildSchema, unknown=fields.Int())

Changes

• src/marshmallow/types.py: Updated UnknownOption type alias to include Field (under TYPE_CHECKING to avoid circular imports)
• src/marshmallow/schema.py: Added isinstance(unknown, Field) branch in _deserialize that routes unknown values through the field's deserialize() method, with proper error collection
• tests/test_schema.py: Added 7 tests covering: basic deserialization, validation errors, Meta config, many=True, load() kwarg, nested schemas, and passthrough with Field()