Make API features queryable

[tomayac]

From: DefinitelyTyped/DefinitelyTyped#73110 (comment):

We should be able to query the API to see if features are available like LanguageModelTool, LanguageModelCreateOptions.responseConstraint, LanguageModelMesaage.prefix.

(CC: @Die4Ever)

[michaelwasserman]

This is a good request; it's hopefully something we can support as a purely-additive enhancement with availability() options

[michaelwasserman]

This seems like a dup of #151, which expresses a plausible shape