Performances: Stop initializing large vector that we’re not going to read

[irevoire]

When streaming a payload with meilisearch-rust, we have to convert a structure implementing AsyncRead to a structure implementing Stream. This means I have to call poll_read, which have a &[u8] in parameter.

Currently, to avoid doing anything wrong, I initialize every byte of my buffer to zero here: https://github.com/meilisearch/meilisearch-rust/blob/437649f05264499de8e0e123e310eeee459bace5/src/reqwest.rs#L159

But ideally, this shouldn’t be necessary since we’re never reading these bytes before writing to it.