IFeatureCollection.Get<TFeature> should be implemented via generic rather than typeof

[benaadams]

Its currently implemented as a typeof wrapper over (TFeature)Get(typeof(TFeature)) which deteriorates more rapidly for the if``else if lookup chain that implementing with generics does.

                          Method |     Mean |         Op/s |
-------------------------------- |---------:|-------------:|
              GetViaTypeOf_First | 12.96 ns | 77,136,481.9 | IHttpRequestFeature 
    GetViaGenericViaTypeOf_First | 22.38 ns | 44,679,211.4 | IHttpRequestFeature 
             GetViaGeneric_First | 19.96 ns | 50,103,862.6 | IHttpRequestFeature
    
               GetViaTypeOf_Last | 69.91 ns | 14,304,944.1 | IHttpSendFileFeature
     GetViaGenericViaTypeOf_Last | 80.70 ns | 12,392,225.5 | IHttpSendFileFeature 
              GetViaGeneric_Last | 33.59 ns | 29,771,919.4 | IHttpSendFileFeature
     
             GetViaTypeOf_Custom | 76.84 ns | 13,014,603.2 |
   GetViaGenericViaTypeOf_Custom | 84.15 ns | 11,882,891.0 |
            GetViaGeneric_Custom | 44.21 ns | 22,617,022.7 |
   
           GetViaTypeOf_NotFound | 76.29 ns | 13,108,494.2 |
 GetViaGenericViaTypeOf_NotFound | 84.44 ns | 11,843,124.1 |
          GetViaGeneric_NotFound | 44.53 ns | 22,454,601.1 |

[muratg]

(Assigning to @halter73 who's already reviewing the PR)