Allow passing a custom serializer for documents

[sanders41]

Pull Request

@LaundroMat FYI in case you want to test this to see if it solves your issue.

Related issue

Fixes #973

What does this PR do?

• Allows passing a custom JSONEncoder to serialize documents with types that the default encoder can't handle.

PR checklist

Please check if your PR fulfills the following requirements:

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

Thank you so much for contributing to Meilisearch!

[sanders41]

Most people will probably not have created a custom encoder before so I added it to the readme.

[sanders41]

bors merge

[meili-bors[bot]]

Build succeeded:

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