codegen: create a file per model

[hughsimpson]

Should address #5287

Relevant changes are clearer after #5366 which commits some incidental refactoring

Adds a new openapiSeperateFilesForModels flag with default false so as not to needlessly break user code (my downstream refactor after enabling this was mostly just find+replace, but still chonky and with edge-cases)

The nifi_test now uses this new flag; it's a pretty large spec, so this provides good coverage, whilst also reducing the number of lines in our expectations (this is the sole reason for the diff having more deleted lines than added).

Writing the output files gets wrapped in a Future.traverse now because writing >1000 model files can be slower than it needs to be, otherwise.

Type aliases are now sorted alphabetically in output.

Inline enums now get written immediately after the 'container' object, rather than at the start of the file. This wasn't a particularly intentional change, but it aligns with the fact that, when 'splitting' to separate files, inline defns will live with the parent. The ordering change should have no impact on user code (although it did cause a bit of churn in test expectations...)

Incidentally fixes an issue with 'deduplicated' file structure when using zio or circe serdes and reusing a json-y Map[String T] or Array[T] type alias