Can Pydantic classes be natively supported without Feature

[dmpetrov]

Description

Feature use to have a good amount of logic in it. Especially related to SQL transpilation:

• _delimiter='__' which become global variable DEFAULT_DELIMITER
• _shallow=False which is not supported now.

Now the transpilation was unified. Only small number of params left:

• _is_file
• _version

It feels like we can get rid of Feature class as well as pydantic_to_feature() that causes multiple issues with multi-processing/threading. We just need to implement all the Feature logic as decorator class (not python-decorator @) instead of parent class. This idea was was briefly discussed in iterative/dvcx#1625

In that way, pydantic's BaseModel would become a feature class.

[dmpetrov]

Closed by #45