Is there some utility that already exists for when I want to get the contents of a file to use as Input<string>? I know about assets.FileAsset but that's not an Input<string>. I can easily write my own but figured that it must already exist somewhere.

Input

is the union of the plain type and

Output

so

Input<T> = T | Output<T>

Which means you can just pass in a plain string to

Input<T>

. So just use whatever lang-specific file-to-string mechanism you would ordinarily use.