Drop GraphQLResolverMap in favor of IResolvers for buildSubgraphSchema

Lots of customers use the `graphql-tools` definition `IResolvers` to specify resolvers rather than the apollo `GraphQLResolverMap` and would like to be able to pass that into `buildSubgraphSchema`. Since one definition is a subset of the other, it should be fairly easy to switch and it won't cause any incompatibilities.

Start by locating the buildSubgraphSchema entry point and its GraphQLResolverMap input, then compare that type with graphql-tools' IResolvers definition. Done means buildSubgraphSchema accepts IResolvers without incompatibilities; check the project's existing type checks or tests for the expected validation.

Written by the indexing model from the issue text.