[WIP] Let users customize their web client

[shimatar0]

Pull Request

Related issue

Fixes #452

What does this PR do?

• This is a WIP state and I'll add it next time.

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• 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!

[shimatar0]

I'll discuss it with @irevoire !

・I want to be able to have a request method in the Client structure to satisfy this issue.
・But request::request is an asycn function, and the signature is very difficult to define

example of an error message

error[E0308]: mismatched types
  --> src/client.rs:54:29
   |
54 |             request_method: request,
   |                             ^^^^^^^ expected struct `Box`, found opaque type
   |
   = note: expected fn pointer `for<'a, 'b> fn(&'a str, &'b str, request::Method<Value, Value>, _) -> Box<(dyn futures::Future<Output = Result<Value, errors::Error>> + std::marker::Send + 'static)>`
                 found fn item `for<'a, 'b> fn(&'a str, &'b str, request::Method<_, _>, _) -> impl futures::Future<Output = Result<_, errors::Error>> {request::request::<_, _, _>}`

[shimatar0]

sorry. I've synced with github(remote) and now I can't get Rebase and push to work, so let me close this PR once.
The new PR will be as follows.
#459