Allow custom base class for models

[Bobronium]

Is your feature request related to a problem? Please describe.
Didn't use this library yet, just looked at code and saw that BaseModel is hardcoded in templates as base class for models:

datamodel-code-generator/datamodel_code_generator/model/template/pydantic/BaseModel.jinja2

Line 4 in f7f0045

class {{ class_name }}(BaseModel):

Describe the solution you'd like
Add ability to choose the name of base class of models (and even create one automatically)
This would be helpful if I wan't to have same config, methods, etc for all subclasses

Describe alternatives you've considered
This can be approached by renaming BaseModel in a result manually, but if it not hard to implement I'd preffered to have this built-in

Additional context
Add any other context or screenshots about the feature request here.

[koxudaxi]

@MrMrRobat
Sounds great to me.

This can be approached by renaming BaseModel in a result manually, but if it not hard to implement I'd preffered to have this built-in

Did you expect to use inherited BasdModel?

I think the code generator should able to receive the alternative base class as arguments or a config file.

[koxudaxi]

@MrMrRobat
support and publish the feature. Thank you

Example

$ datamodel-codegen --input ./tests/data/api.yaml --base-class custom_module.CustomBase
Output:

class Pet(CustomBase):
    id: int
    name: str
    tag: Optional[str] = None
...

[Bobronium]

Looks great, thank you!