Inheritance refactoring

[curquiza]

Currently, the sens of inheritance is (from parent to children classes)
Document, Settings... -> Index.
And: HttpReqest is used as a module

What we should have:
HttpRequest -> Index and Client

This is more logical because Index and Client are classes to communicate with MeiliSearch, so it should inherit the HttpRequest properties to actually do this communication.
This way, we could avoid a lot of duplicated codes, specifically with the config parts. When instantiating Index, the parent constructor (HttpReuest one) would be called, and they will share the same configuration without passing config everywhere.