[ofrep] add cache handling

[thepabloaguilar]

As the OpenFeature Remote Evaluation Protocol - API Specification specifies an extension endpoint to get the provider configuration where it returns the cache polling interval it'd great to handle this in the ofrep client to have a local cache strategy which is only refreshed when the time expires!

[Kavindu-Dodan]

@thepabloaguilar thank you for bringing this up. There's an ongoing discussion dedicated to OFREP server providers [1]. We should be able to add caching support and improve GO OFREP provider once this discussion is resolved.

[1] - open-feature/protocol#17