add feature for clients that do not implement send

[ezegros]

Pull Request

Related issue

Fixes #577

What does this PR do?

Some clients do not have Send compatible futures. To fix this I propose adding a feature disabled by default to support this kind of clients like acw, Send should be supported by default.

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)?
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Thank you so much for contributing to Meilisearch!

[irevoire]

Hey @ezegrosfeld, thanks a lot for your contribution.

It’s exactly what I was looking for!

bors merge

[meili-bors]

Build succeeded:

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