Add config option to set enum case

[BreakBB]

With #282 the casing of generated Enums was changed from some broken/inconsistent state to using PascalCase as default. While the TypeScript docs also uses PascalCase in the examples, I think it would be good to have a config option, to change it.

The TypeScript generators listed on openapi-generator.tech all have an enumPropertyNaming option which allows:

• camelCase
• PascalCase
• snake_case
• UPPERCASE
• original

From my understanding this is the place where the config value should be read and matching formatter should be called:

https://github.com/fabien0102/openapi-codegen/blob/4c0f66935a4cf299ce9818be22f9ff6acbc39158/plugins/typescript/src/core/schemaToEnumDeclaration.ts#L79

Also the filenameCase option already have more or less the values this new config option would need:

https://github.com/fabien0102/openapi-codegen/blob/4c0f66935a4cf299ce9818be22f9ff6acbc39158/plugins/typescript/src/generators/types.ts#L27

Overall this shouldn't be a complicated but much appreciated change.

Please let me know if you need any more information on this.