feat: introduce OFREP provider

[Kavindu-Dodan]

This PR

Fixes #473 and introduce the Go OFREP provider 🎉

Usage

Consider the following code snippet for basic usage,

provider := ofrep.NewProvider(
                     "http://localhost:8016",
                      ofrep.WithBearerToken("TOKEN"))

Implementation

This implementation relies on OFREP single flag evaluation endpoint. And there is no caching layer at the current implementation and flag evaluations always rely on remote evaluation results.

I have introduced modules where appropriate which should allow us to expand the implementation with a caching layer and other OFREP extensions that we plan to introduce

[lukas-reining]

Looks good to me!
All seems as we planned it for JS and I can not find any issues.

[thomaspoignant]

Hey @Kavindu-Dodan great work, just one quick question.
I see a lot of similar code, and I was wondering why you did not use generics since you are using go1.21?
It will make the provider maybe a bit easier to understand.

What do you think?

[thomaspoignant]

I also think that you don't have implement the retry-later header when we receive a 429

[Kavindu-Dodan]

@thomaspoignant thank you for reviewing the PR.

I see a lot of similar code, and I was wondering why you did not use generics

I assume you are referring to flags.go [1]. Yeah, it seems to have repeated logic differed by returned type, but each handler has value conversions and a specific return type of struct. I had a quick look into refactoring them but I couldn't find any good way of doing that.

note - checked JS implementation, which use generic ResolutionDetails. unfortunately, go doesn't use such a generic type

I also think that you don't have implement the retry-later header when we receive a 429

For Go SDK, I focused on the dynamic context paradigm, hence there is no support for bulk evaluation. So to handle 429, I think what we should do is to introduce a request blocker with a timer [2]. I haven't done it in this PR.

[1] - https://github.com/open-feature/go-sdk-contrib/pull/477/files#diff-df3e1ced5c1dbe3274ff2d4cc8a47eaf0aab9475b4f526501c30491cb9d499ca
[2] - https://github.com/open-feature/go-sdk-contrib/pull/477/files#diff-4f6c2e021ab239118a7327dccc6fcab63a9f8480d79e34f57006ad7114fb463eR69-R70

[thomaspoignant]

👌