Consider the ability to allocate variants based on custom filters

Variants are going to be a huge boon to this library and I'm excited to leverage them. I often find myself creating custom filters that evaluate based on the application state rather than the traditional user centric targeting scenarios. It would be great if we could allocate variants based on these filters. Perhaps something like:

  "FeatureManagement":
  {
      "MyVariantFeatureFlag":
      {   
          "Allocation": {
              "DefaultWhenEnabled": "Small",
              "AllocatedFor": [
                  {
                      "Name": "CustomFilter",
                      "Parameters": {
                          "SomeParam": "SomeValue"
                      },
                      "Variant": "Big"
                  }
              ]
          },
          "Variants": [
              { 
                  "Name": "Big"
              },  
              { 
                  "Name": "Small"
              } 
          ]
      }
  }

Start by reviewing the variant allocation and custom-filter behavior described in issue #460, then trace the existing variant and filter entry points in the repository. Define how the shown FeatureManagement configuration should select Big versus Small for application-state filters, and add coverage for the requested configuration and selection behavior.