Fix support for custom similarities

[gmarz]

Currently, there's no way to specify a custom (plugged in) similarity via the fluent syntax. See https://discuss.elastic.co/t/custom-similarities-in-elasticsearch-nest-2/56496/6

Also, looking at SimilarityJsonConverter, we have an ugly hack where we're resolving the similarity type via type name. This would blow up when deserializing a custom similarity unless it was under the Nest namespace.