[Generator] Multipart support

[czechboy0]

Motivation

Implement SOAR-0009. Fixes #36.

Modifications

It's a large diff, and adds some complexity, however I tried to avoid needlessly complicating existing code and opted for duplicating parts of the code specifically for multipart, for easier reasoning.

The implementation very much maps to the changes described in the proposal, which I won't repeat here.

Some notable highlights before you dive in:

• Testing is mainly done through snippet tests, and I added one request and one response operation to make sure things work end to end. Maybe review those first.
• The logic for generating request and response bodies got slightly extended to detect when we're generating multipart content, and branches off to bespoke code specifically for multipart, all in the new Multipart directory.
• Made a few related changes, but tried to keep this isolated to multipart as much as reasonable.
• Let me know which parts I should elaborate on, I'm happy to explain in more detail.

Result

Multipart content now works, as proposed in SOAR-0009.

Test Plan

Added a bunch of snippet tests and two new file-based reference test operations, including petstore consumer tests to verify that it all works at runtime as well.

[simonjbeaumont]

👏 Great stuff.