Be consistent in blocking provider API function (async param vs `...AndWait` name)

We should improve the consistency of the function for setting a provider and blocking. In some cases we have an `async` param, and in others we use an `...AndWait` function name.

## Background:

> I dislike that we mix `...AndWait` and `aysnc bool` in our public API - it's inconsistent - but this isn't something you introduced, it already existed elsewhere. I think in v2 we should go for one or the other (the spec doesn't dictate function naming so we have flexibility here).

_Originally posted by @toddbaert in [#442](https://github.com/open-feature/go-sdk/pull/442/files#r2461023401)_