Add Facet search support

[danFbach]

Pull Request

Related issue

Fixes #461

What does this PR do?

• implements facet search

PR checklist

Please check if your PR fulfills the following requirements:

• Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
• Have you read the contributing guidelines?
• Have you made sure that the title is accurate and descriptive of the changes?

Few notes:

• Implemented the FacetSearchAsync method with a facetName parameter seperate from a FacetSearchQuery object parameter, similar to how SearchAsync separates out the query parameter from the SearchQuery object, since these parameters are required.
• Not sure how we want to handle an empty or null facetName? This may change tests requirements. See commented out tests
• Test FacetSearchWithFilterFacetIsNull is "wrong." Perhaps this is just how meilisearch works, but filtering by genre IS NULL returns nothing, but 2 entries in the IndexForFaceting-SearchTests have a null genre. Would like some feedback here
• Any additional tests?

[curquiza]

bors merge

thank you a lot @danFbach

[meili-bors]

Build succeeded:

• format-check
• integration-tests