[FEATURE] Generics for v2 release of go-sdk

### Requirements

Now the go-sdk has been upgraded to go 1.18 it can make use of generics.

The obvious candidates where leveraging generics would reduce bloat:

- [Evaluation types](https://github.com/open-feature/go-sdk/blob/main/pkg/openfeature/client.go#L137) are effectively the same structure with different `Value` types
- [Resolution types](https://github.com/open-feature/go-sdk/blob/main/pkg/openfeature/provider.go#L76), as above, are effectively the same structure with different `Value` types

Pros:
- Less types to maintain
- Less duplicated code
- Less type inference
- Less chance of accidental inconsistent behaviour across types

Cons:
- Breaking change
- Breaks contract between providers and sdk

Perhaps this undertaking isn't worth the effort but it's at least worth raising.

Start by reading the evaluation types in pkg/openfeature/client.go around line 137 and the resolution types in pkg/openfeature/provider.go around line 76. Determine whether replacing these duplicated value-specific types with generics is appropriate for the v2 release, including the provider-to-SDK contract. The issue does not define further acceptance criteria.

Written by the indexing model from the issue text.