UserStore does not update tokens when SetTokenAsync() is used

[martincostello]

By default, the base implementation of UserStoreBase<T> in ASP.NET Core Identity, which the UserStore<T...> classes derive from, does not write to the underlying data store by default if a token being set already exists in the store as it assumes change-tracking is used.

Depending on the outcome of dotnet/aspnetcore#47306, to prevent other consumers of this library from falling into the same pit of failure, it would be worth considering changing the base implementation in this library to do the following instead:

public virtual async Task SetTokenAsync(TUser user, string loginProvider, string name, string? value, CancellationToken cancellationToken)
{
    cancellationToken.ThrowIfCancellationRequested();
    ThrowIfDisposed();

    ArgumentNullException.ThrowIfNull(user);

    var token = await FindTokenAsync(user, loginProvider, name, cancellationToken);

    if (token is null)
    {
        token = CreateUserToken(user, loginProvider, name, value);
    }

    await AddUserTokenAsync(token);
}