codegen: various minor fixes and improvements

[hughsimpson]

A few sundry things:

• The discriminator of each child of a oneOf is reified as def $discriminatorName: String = "...", but the parent type didn't declare the abstract method, which was irritating. Parent will now declare the method.
• Fixes a reference bug when inheriting an enum using circe serdes, where parent didn't use enum in json bodies but the child does, and seperateFilesForModels=true
• Adds a openapiAlwaysGenerateParamSupport boolean flag, so that enums can be defined in an inherited openapi and used in path position, without having to redeclare them (and everything that transitively contains them)
• Fixes client encoding of non-exploded array params (was using a .map instead of a .flatMap and producing e.g. ?myParam=List(uuid1),List(uuid2) instead of ?myParam=uuid1,uuid2 when encoding)
• Remove redundant backticks from discriminator method names when the property name is not a reserved name
• Tries harder to construct type aliases for inline definitions when 'deduplicating' between schemas. It doesn't work as it should but it's better
• Creates val aliases for 'dedupped' case class definitions

[hughsimpson]

I don't want to deal with merge conflicts on the other prs 😅 so I'm leaving as a draft for now

EDIT: OK rebased now

[hughsimpson]

The def isn't generated here because the property name is noMapType on this oneOf and the children don't declare that field. I think that's strictly speaking invalid openapi but anyway...

[adamw]

Thank you :)