Openapi stuff

[olydis]

• fixes nested (inline) schemas don't honor required properties autorest#2688
• fixes a leftover "required-ness corresponds to usage, not definition" bug
• normalizes order of stuff (reponse status codes, ...) => more stable code generation
• fixes required single-value enum body-parameter non-constantness (was mistakenly not modeled as constant before); fixes required single-value enum body parameters aren't modeled as constant autorest#2689

[olydis]

🤖 AutoRest automatic publish job 🤖

> will publish once PR gets merged

[olydis]

no changes across the entire specs repository, apart from:

• required-ness change (see above, "quirks mode" via directive trivial to implement)
• request content type change for body-less requests not maintained (since no way to encode in OpenAPI 3.0 - content-less requests simply have no content type)