Consider breaking up primary interfaces, or not exporting them

> Remove (or unexport) certain identifiers (types, functions, methods, interfaces) that are unnecessary. For example, [openfeature.IEvaluation](https://pkg.go.dev/github.com/open-feature/go-sdk/openfeature#IEvaluation) or [openfeature.IClient](https://pkg.go.dev/github.com/open-feature/go-sdk/openfeature#IClient) should most definitely not make it into v2. We will want to be especially careful about which interfaces we are exporting. Those that are exported must be named appropriately (not like IClient or ifaceClient or ClientIface).

We need to discuss this further, but we should rethink how we expose these interfaces.