First class field validator

Documenting in person discussion.

It might make sense to have a "type": "field" schema or something along this lines to collect options that apply to the field and not the type of the field, like a "not required" optional that would leave the field unpopulated if it is not included.

[adriangb]

I would also lean towards one single syntax: I imagine these schemas are (mostly) going to be programmatically generated, so having shorthand is only really helpful for writing tests for this package.

[samuelcolvin]

From https://github.com/samuelcolvin/pydantic-core/pull/92#pullrequestreview-1012497346

We want an intermediate dict above schema on a field. This does not affect the rust validators or validation, just the python interface for building schema.

Example:

{
    'type': 'model',
    'fields': {
        'field_a': {
            'required': True,
            'schema': {'type': 'str'},
        },
        'field_b': {
            'default': 123,
            'alias': ...
            'schema': {'type': 'int'},
        },
    }
}

We might add other stuff beyond required, schema, alias.