Add pearsonr mc, update functionality to match the original docs
_The documentation is not available anymore as the PR was closed or merged._ thank you @lhoestq!! :hugs: 
- adds pearsonr metric card
- adds ability to return p-value
    - p-value was mentioned in the original docs as a return value, but there was no option to return it. I updated the _compute function slightly to have an option to return the p-value.