[Core, Rust Server] anyOf / oneOf support for Rust Server

[richardwhiuk]

This adds anyOf/oneOf support for the Rust Server generator using the support added in Metaswitch/swagger-rs#112

There's a couple of minor core changes to InlineModelResolver - @OpenAPITools/generator-core-team:

• If we get a model named 12345AnyOfObject, rename it to _12345AnyOfObject, not _2345AnyOfObject.

  This means for Rust Server, it'll end up as Model12345AnyOfObject, instead of Model2345AnyOfObject, which is nonsensical.

  ⚠️ I think this is technically breaking, but it's clearly bugged at the moment.

• Perform inline model resolution for enums, and pass the type through when performing inline model resolution.

There's a couple of minor fixups in Rust Server:

• Some minor template tweaking, including putting the IntoHeader implementation below the model definition.

• Update the supported features - we've support XML Structure Definition for a while - but the global feature flag is wrong - see user feedback in [REQ] [rust] Support XML objects in requests and responses #5594.

• Handle FromStr properly, which breaks for the oneOf model otherwise.

Optimistically, I'd like to merge this before 5.0 gets cut.

This change is by @mirw and myself on behalf of @Metaswitch.

Rust Server Technical Committee

• @frol
• @farcaller
• @paladinzh

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project beforehand.
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[richardwhiuk]

CircleCI passsed, but seems to have failed to sync with GitHub - https://app.circleci.com/pipelines/github/OpenAPITools/openapi-generator/8310/workflows/54e31e58-6aab-45e7-83f9-4b41106f8790