[R] return API response when content is raw (not coercible to text)

[mattpollock]

This is a new PR for a breaking change removed from #20131.

@Ramanth, @saigiridhar21, @wing328

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[mattpollock]

@wing328 feel free to keep or discard this change. As requested I pulled this out of #20131. If declined, the result will be that when an endpoint that returns binary data (e.g., a gzipped directory) that cannot be coerced to text, the return value will be a raw vector. If accepted, the return value in this case will be an instance of ApiResponse, which is what gets returned if there are error codes too.

[mattpollock]

Another alternative would be to return(invisible(local_var_response$content)) so that the raw vector, though returned, will not be printed in the event that it is not captured to a variable. This may be a better option since it seems likely that these sorts of endpoints will be invoked without assigning the response to a variable.