Add a payload size limit on search route

[ManyTheFish]

It's possible to make malicious search queries by sending oversized payloads, which will consume all meilisearch resources (DDOS).
For example, sending a word containing 1M a makes meilisearch work for 5s.

We should add a payload size limit on the search request which would be different than the payload size on document routes.

related to meilisearch/meilisearch#2175