Allow annotation of tag fields

[umarbutler]

Description

Currently, because tag_fields cannot live directly underneath a Struct as attributes but instead are defined as arguments of Struct, it does not seem possible to annotate tag fields such that, when exporting an OpenAPI schema, descriptions and examples for a tag field could be included.

This is unfortunate as I am trying to use msgspec in a production application alongside litestar to create an extensively documented autogenerated OpenAPI schema for consumers of my API and not having the ability to annotate tag fields will make my schema incomplete.