Add a header for autogenerated classes

What happened?

This issue is linked to https://github.com/palantir/conjure-python/issues/595. When I discovered an issue with the palantir-python-sdk library, it was pretty much impossible to know that the bug was originating from conjure-python until the person who owns the project told me so.

Additionally, after a bug is fixed in this library, there's currently no way to know if a class generated by this library may be impacted by that bug.

What did you want to happen?

Projects that generate code usually include annotations or a header, so that it's clear how this code is generated (examples that come to mind are jOOQ / Immutables) Ideally, this should include a version number, so that it's easier to read changelogs and understand any potential impact.

Here's what a simple implementation could look like:

'''
This file has been generated by conjure-python version x.y.z
https://github.com/palantir/conjure-python
'''

import ...

class TestService(Service):

Trace how conjure-python emits generated Python classes and where the generator version is available. Define the header format from the issue, including the version and project URL, then verify that generated classes consistently include it and that the result remains valid Python.