Add 'x-generate-alias-as-model' extension to allow enabling generating alias as model per-schema

[bkabrda]

This allows turning on the generate-alias-as-model option per schema with an x-generate-alias-as-model extension.

Since this is not a language-specific change, but rather a core change, I'm not sure who to CC... So hopefully CCing @wing328 and @jimschubert is ok.

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[wing328]

cc @OpenAPITools/generator-core-team

[wing328]

LGTM. Will merge tomorrow if no further feedback from the core team.