Update FilterCollectionExtensions

[zhiyuanliang-ms]

The usage of AddForFeature is like:

services.AddMvc(o => 
{
    o.Filters.AddForFeature<ThirdPartyActionFilter>("MyFeatureFlag");
});

So it makes more sense to return FilterCollection to support the following usage:

services.AddMvc(o => 
{
    o.Filters.AddForFeature<ThirdPartyActionFilter1>("MyFeatureFlag1")
                .AddForFeature<ThirdPartyActionFilter2>("MyFeatureFlag2");
});

[rossgrambo]

This is a "breaking" change so I'm not sure if we can do it. The reason "breaking" is in quotes is because it looks like we only ever return null here... But in the rare case where someone has:

services.AddMvc(o => 
{
    IFilterMetadata metadata = o.Filters.AddForFeature<ThirdPartyActionFilter>("MyFeatureFlag");
});

We would break them.

[zhiyuanliang-ms]

@jimmyca15 How about make this PR targeting on preview branch to avoid making breaking changes on our current stable release?