Use a class variable to store linked data in Pydantic models in `dandischema.models`

[candleindark]

The linked data of each model defined in dandischema.models is currently stored with the private attribute _ldmeta, such as https://github.com/dandi/dandi-schema/blob/6d6c7b18addc40bbbbe8311d4a3f8a37e832cc3d/dandischema/models.py#L645C5-L645C34. We can instead store the linked data of each model in a class variable instead.

Specifically, instead of the following

_ldmeta = {"nskey": "schema"}

we can have

ldmeta: ClassVar[dict] = {"nskey": "schema"}. # Using an identifier without an underscore

Doing so has several benefits.

1. Semantically, a class variable relates to the containing class/model while a private attribute relates to a instance of the class. Thus, a class variable is more in line with the fact that the link data relates to the model not to an instance of a model.
2. We don't have to use an identifier that starts with an underscore. (We can use just ldmeta or even linked_data)
3. The assigned value will not be implicitly coerced into another type.
   Currently the initial assigned value of a _ldmeta attribute is coerced into a pydantic.fields.ModelPrivateAttr, as depicted below.

    from dandischema.models import BaseType
    type(BaseType._ldmeta)
    <class 'pydantic.fields.ModelPrivateAttr'>

   As a result, to access the initial assigned value is rather cumbersome as https://github.com/dandi/dandi-schema/blob/6d6c7b18addc40bbbbe8311d4a3f8a37e832cc3d/dandischema/metadata.py#L75C12-L75C49.

Please let me know if I should proceed with this proposed change with a PR.

[yarikoptic]

@satra - where having this _ldmeta manifests ATM? we do have https://github.com/dandi/schema/tree/master/terms but it was not updated for 5 years and unclear where we use it already if we do, or at least where we serialize it?

@candleindark one way or another, we should incorporate this in produced linkml models.