Switch to a fixed HTTP client library OkHttp #419

Description
Following this issue #334 and the fact that the PATCH method is now needed in multiple methods since the v0.28.0 of Meilisearch.

Expectations
The need to allow the PATCH method and to keep the Java 8 compatibility pushed us to choose as a solution to use a single HTTP library which in addition to meeting the needs would reduce the dependencies.
OkHttp has the advantages we were looking for it is a very used library that supports PATCH.

What to do?

• Remove all Java HTTP Client to only keep OkHttp
• Rewrite the Meilisearch HTTP client to simplify it