Add access to response options similarity

## Description

Please add access to the similarity matrix for response options in the R library.

## Rationale


Alex Nikic @AlexNikic has made a PR in the Python library and API which exposes similarity of response options. We need to expose this in R.
 https://github.com/harmonydata/harmony/pull/101
https://github.com/harmonydata/harmonyapi/pull/20