Add generator helpers

[RobertCraigie]

Problem

Creating custom Prisma generators in python requires a lot of knowledge on the internal workings of Prisma and a considerable amount of boilerplate, we should add helpers that will make custom python generators easy.

Suggested solution

from prisma.generator import BaseGenerator, Manifest, Data

class MyGenerator(BaseGenerator):
    def get_manifest(self) -> Manifest:
        ...

    def generate(self, data: Data) -> None:
        ...

MyGenerator.invoke()

Additional Context

We should also refactor the internal generator to use this helper as well so that custom generators can make use of our utilities such as rendering templates and generation safety.