Move feature implementations into `Microsoft.AspNetCore.Http.Features` namespace

[dougbu]

#589 exposes all of the feature interfaces publicly. But the feature implementations remain in Microsoft.AspNetCore.Http.Features.Internal and Microsoft.AspNetCore.Http.Features.Authentication.Internal. Since the feature implementations are used in Kestrel and supported, they should also be moved to Microsoft.AspNetCore.Http.Features and Microsoft.AspNetCore.Http.Features.Authentication.

See comment here for example.

[dougbu]

Self-assigning because I was playing with this cleanup already.