Support Generating IndexMap Instead of HashMap

[JosiahBull]

For some usecases, e.g. snapshots in tests, consistent ordering via IndexMap would be preferred over a randomly ordered HashMap in generated clients.

I am happy to do the work required to allow users of the progenitor library to opt-in to this feature via a feature flag or as the default. I can alternatively extend support this feature via the x-rust-type mechanism, with a special case when path is equal to indexmap::IndexMap or indexmap::IndexSet to enable this feature.

Let me know if this is in-scope for the features you'd like progenitor to support and I'll open a PR. 👍

[JosiahBull]

Just found this related issue: oxidecomputer/typify#555

Would support be welcome all the way through into Progenitor, or only in Typify?

[ahl]

For sure. Plumb it right on through!