[FEATURE] Create OFREP provider

[thomaspoignant]

Requirements

OFREP (OpenFeature Remote Flag Evaluation Protocol), is an API specification for feature flagging that allows the use of generic providers to connect to any feature flag management systems that supports the protocol.

We would like to have an OFREP provider in Elixir.
The Elixir SDK is a server SDK, so we should follow those guidelines when building the provider.