Generate @frozen enums for operation outputs and request/response bodies

[simonjbeaumont]

Motivation

The generated code contains several enum types. A lot of these are just namespaces (i.e. the enum has no cases), but some are "proper" enums that we expect adopters to switch over. For example the operation output, and the request and response bodies.

To make using these types more ergonomic, we can annotate them as @frozen. This is something we already do for other generated enum types (e.g. OneOf and enum types declared in the OpenAPI document).

Modifications

• Add desired @frozen attributes to reference code
• Generate @frozen enum for request bodies
• Generate @frozen enum for response bodies
• Generate @frozen enum for operation outputs

Result

Switching over operation outputs and request/response bodies is simpler.

Resolves

• Resolves Mark operation output and request/response body enums as @frozen  #105.

Test Plan

Reference test updated.

[simonjbeaumont]

I'm going to move this to draft and mark as blocked for now because there's some more discussion that maybe this is the wrong direction to be moving in for the generated code.

[czechboy0]

lgtm on the implementation, but as you say, this is still being discussed elsewhere.

[czechboy0]

@swift-server-bot test this please

[czechboy0]

Updated from semver/minor to semver/patch, as this isn't a breaking change.