Add async search with relevance score

[jpzhangvincent]

Your PR Title (What it does)

Add the async version for the search with relevance score

Before submitting

Any suggestion to fix the linting issue? Thanks!

Who can review?

Community members can review the PR once tests pass. Tag maintainers/contributors who might be interested: @dev2049 @hwchase17

[jpzhangvincent]

It seems there are some linting problems with the output type since the "relevance_score" method returns the tuple. What's the best way to fix that?