[typescript-fetch] Fix API-generation of referenced enums for multipart/form-data

[Masamas]

Adds isEnumRef to the condition because it does not match the isPrimitiveType condition if the enum definition is separated. isEnumRef was also added to the typescript-fetch specific ExtendedCodegenParameter and ExtendedCodegenProperty so that it can be used in templates.

Fixes #11613

@TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @topce @akehir @petejohansonxo @amakhrov @davidgamero @mkusaka

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