fix!: Rename namespace from OpenFeature.SDK to OpenFeatureSDK

[benjiro]

Discovered when implementing a provider, I noticed due conflict with OpenFeature.SDK namespace and the singleton class being called OpenFeature.
It requires the full namespace to use the OpenFeature singleton.

Avoid this by making the root namespace OpenFeatureSDK instead of OpenFeature.SDK

An example of the issue

After the proposed change

using OpenFeatureSDK;

OpenFeature.Instance.SetProvider(new CloudBeesProvider());
services.AddSingleton(provider => OpenFeature.Instance.GetClient());

[toddbaert]

I would say this is a fix, which will include it in the changelog. I recommend you change the PR title.

I updated the title. If you disagree feel free to revert.