Dictionary properties which have IUrlParameters types as keys should have ResolvableReadOnlyDictionaryConverter applied

[stevejgordon]

Any properties on generated types which are defined as a Dictionary<TKey, TValue> where TKey is an IUrlParameter require special handling to support consumer lookups. GetFieldMappingResponse.FieldMappings is an example where the key is Field. In such cases, if a consumer attempts to lookup using a Field inferred from a type, no matches will be found by default. We need to apply a specialised dictionary which sanitises the keys to a normalized form using the IElasticClientSettings.

We will need to redefine these properties as IReadOnlyDictionary and apply a specialised converter to handle deserialization.