[js/plugins/anthropic]: Add support for structured responses

In the new js anthropic plugin, we should support structured responses as the anthropic SDK/API does

> Anthropic SDK supports JSON schema output format:
https://github.com/anthropics/anthropic-sdk-typescript/blob/3852b086203f9d7795c5c9e288fba992fc17b61b/examples/parsing-raw.ts#L11


As part of this work, we should add an example testapp for structured responses.