[Protobuf-Schema] Add enum as model support

[Yurzel]

resolves #10867

Resources:

• Added enum.mustache
• Partial render to model.mustache.

Codegen

• Add "protobuf-enum-index" to enumVars in model aswell (needed for field numbers)
• Set ClassName to match the enumName when used as CodegenProperty

[Yurzel]

Had to revert ClassName modification, because name didn't match the reference in message. Model inside postProcessOperationsWithModels doesn't have isEnum set to true so I decided to go this way.

[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[Yurzel]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

Thanks for the info. I hope I did it properly. Did it for the other branches aswell. #10892 and #10893.

[wing328]

LGTM.