Add support for VAMANA index algorithm at index creation (#430)

* - add structured object-model for complex vector index creation, with specialized types for the 3 vector types
  - add VectorField.Dimensions, .Type and .DistanceMetric
  - add mechanism to bake those into command without breaking existing usage
    - AddDirectAttributes, DirectAttributeCount
  - new type FlatVectorField
  - new type HnswVectorField
  - new type SvsVanamaVectorField
- make it possible to extract approx command from SerializedCommand.ToString()
- add implicit operator from string to FieldName to reduce new overloads needed
- add new Schema.Add*VectorField APIs
- add tests to demonstrate correct index construction

* dotnet format

* fix broken test (missing <index_attribute_count>)

* PR nits

* - add SVS integration tests
- fix LeanVec command typo

* follow-up typo fix

* Comments: clarify compression/reduce interaction

Author: mgravell

src/NRedisStack/Search/FieldName.cs

@@ -36,5 +36,7 @@ public FieldName As(string attribute)
             this.Alias = attribute;
             return this;
         }
+
+        public static implicit operator FieldName(string name) => Of(name);
     }
 }