Add a header on all files indicating they've been generated

[ktekelioglu]

Before this PR

Fixes https://github.com/palantir/conjure-python/issues/596

After this PR

==COMMIT_MSG== Add a header on all files indicating they've been generated by conjure-python ==COMMIT_MSG==

Possible downsides?

[changelog-app[bot]]

Generate changelog in changelog/@unreleased

Type See change types. Select one:

• [x] Feature
• [ ] Improvement
• [ ] Fix
• [ ] Break
• [ ] Deprecation
• [ ] Manual task
• [ ] Migration

Description Add a header on python files indicating they've been generated by conjure-python

Check the box to generate changelog(s)

• [x] Generate changelog entry