Code-First Federation 2 Support

[Shane32]

Based on #3144 by @Mithras

• New code is in namespace GraphQL.Federation
• Old code in namespace GraphQL.Utilities.Federation has been marked as obsolete
• Federation code is unified into .AddFederation() for both schema-first, code-first and type-first use cases.
• Old schema-first syntax is marked as obsolete, but will function without any (or very few) changes
• Code first supported, mostly by @Mithras
• Type first supported
• Apollo Router now runs an actual test against four live subgraph schemas (aka demo apps) to form a single supergraph:
  • Schema first, new syntax, federation 1 compatible
  • Schema first, old syntax, federation 1 compatible
  • Code first, federation 2 compatible
  • Type first, federation 2 compatible

Breaking changes:

• A couple classes were moved.
• The __typename field is not implicitly added to federation requests

Prerequisites:

• Add another Modify overload to GraphQLAttribute #3952
• Add IsPrivate and SkipTypeCheck properties to IGraphType / IFieldType #3949
• Add ISchemaNodeVisitor.PostVisitSchema #3951
• Applied directives can contain metadata #3965

Split into separate PRs:

• Add GraphQL Federation v2 required types #3957
• Add federation metadata extension methods and attributes #3958
• Fix RemoveFederationTypesVisitor #3961
• Revamp federation resolvers #3960
• Refactor IFederationResolver; add FederationResolverAttribute #3963
• [WIP] Add federation builder extension and related schema configuration code #3964
• Add sample applications / tests

To-do / nice-to-have:

• Analyzer rule for new [FederationResolver] attribute to ensure it is used correctly
• Simplify defining federation keys #3962
• New tests for Sample3 and Sample4 web applications
• Additional federation testing for Sample3 and Sample4
• Move new extension methods to GraphQL.Federation namespace rather than GraphQL.Federation.Extensions
• Add documentation
• Break PR into smaller pieces and merge each piece separately
• Merge master/develop into PR
• Add xml comments to all federation classes/etc

Moved out of scope:

• New validation rule - see Add 5.2.4 Operation Type Exists graphql/graphql-spec#1098
• Consider: auto-hide any unreferenced types in the schema (??)
• Consider: post-initialization schema visitor