ORM: support json

We are not unpacking JSON by default -- we should consider doing that.

Pydantic supports JSON unpacking (partial & full), details here: https://docs.pydantic.dev/latest/concepts/json/#json-parsing

-----------------

If we have this in the schema

```esdl
type User {
name: str;
data: json;
}
```

we can maybe enable something like this then:

```python
# User subclasses the generated model:
class MyUser(models.default.User):
data: gel.JsonUnpacker[
models.default.User.__typeof__.data,
MyDataModel
]

# User defines their model to parse JSON:
class MyDataModel(pydantic.BaseModel):
...
```

Or, alternatively:

```python
# User subclasses the generated model to
# specify a *new* computed field to unpack
# JSON into
class MyUser(models.default.User):
parsed_data: gel.JsonUnpackFrom[
models.default.User.__typeof__.data,
MyDataModel
]

# User defines their model to parse JSON:
class MyDataModel(pydantic.BaseModel):
...
```

I think we have two options:

1. Always unpack JSON -- if there's a type for it setup -- use that. No type -- return whatever `json.loads()` returns

2. Only unpack JSON when the user asked for it, either with ORM model customization, or with the upcoming `.with_codecs()` API.

I'm in favor of (2).