Add support for proximity precision setting

[william-des]

Pull Request

Related issue

Fixes #517

What does this PR do?

Add the new proximityPrecision setting with get, update, and reset methods associated.

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?

Thank you so much for contributing to Meilisearch!

[curquiza]

bors merge

[curquiza]

thank you @william-des

[meili-bors]

Build succeeded:

• format-check
• integration-tests