Expose format field for string configurable props (#273)

Configurable props of type `"string"` can have a `format` field that
specifies the expected format of the string. Currently, the only
supported format is `"file-ref"`.

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Co-authored-by: fern-api[bot] <115122769+fern-api[bot]@users.noreply.github.com>
Co-authored-by: js07 <19861096+js07@users.noreply.github.com>
Co-authored-by: Jay Vercellone <jay@pipedream.com>