Provide two methods to add documents from an async reader

[irevoire]

Pull Request

When using Meilisearch-rust to forward documents from another source it’s a pain to use.
These two simple methods let you send raw payload to meilisearch. But you need to specify your content-type.

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## Note

I didn’t implements these method for the wasm version of the crate because wasm-bindgen-futures don’t provide an AsyncReader trait.

[bidoubiwa]

Looks good to me. For the clippy issue I think you just need to rebase with main

[bidoubiwa]

bors try

[bors]

try

Build failed:

• clippy-check

[bidoubiwa]

bors try

[bors]

try

Build failed:

• integration-tests

[bidoubiwa]

bors try

[bors]

try

Build succeeded:

• integration-tests
• clippy-check
• rust-format
• readme-check
• wasm-build

[bidoubiwa]

bors merge

[bors]

Build succeeded:

• clippy-check
• integration-tests
• readme-check
• rust-format
• wasm-build