Implemented fragments shared between multiple queries

[FabioStatespace]

This has no priority right now but it may be useful in the future to differentiate only some parts of some queries between platforms using Fragments

• .graphqlfrag files are parsed as GraphQLFragmentFile that contain a Fragment definition
• It's possible to add fragments in GraphQLConfig that are loaded in a Dictionary in GraphQLClient
• Queries now contain a list of all Fragment names that they references, that list can be used to get Fragments from the graphql client
• Query.ToRequest accept an IEnumerable to append Fragments to the query

To make it work in addition to this PR it requires to change the request creation in GraphQLUtility:QueryInternal like this:

var fragments = _client.FindFragments(query.Fragments);
var request = query.ToRequest(args, fragments);