plugin-client: Add auth

[nimo23]

The https://kubb.dev/plugins/plugin-client/ has no built-in support for authentication. It would be nice to have auth similar to https://heyapi.dev/openapi-ts/clients/fetch#auth.

[HelenaVeltri]

You're right. I was having a problem with auth with the client plugin, but you can create our own auth logic and config with a .ts file and import to your client plugin as importPath, it helped resolve my problem but it's much more difficult than just using a build-in support for it :/