[v1.11.0] Support facet distribution for federated search

[brunoocasali]

Following this central issue

Explanation of the feature

Related issue in the engine: meilisearch/meilisearch#4932

• Request facet distributions and facet stats in your federated search, by using federation.facetsByIndex in the POST POST /multi-search route.
• Merge the returned facets in a single facet distribution and stats that is global to the entire request, by using federation.mergeFacets in the POST POST /multi-search route.