[Meilisearch v1.40] Add support for `distinct` in federated search requests

[Strift]

Context

Meilisearch 1.40.0 introduces support for distinct in federated search requests.

[!note] A federated search is a multi-search request using the federation option.

Objective

We want to update the multi-search method(s) to accept an optional distinct string in the federation options.

Tasks

• Update multi-search API method(s) to accept an optional distinct attribute in federated search
• Add test cases for the federated search request using distinct