Allow passing Map of DataFetcher registrations to SchemaMappingInspector as alternative to RuntimeWiring

[rstoyanchev]

Currently the inspector expects the GraphQLSchema and RuntimeWiring. This works fine for our purposes since we use RuntimeWiring to addDataFetcher registrations. However, instead of performing direct registrations, alternative GraphQlSource.Builder implementations may also use the GraphQLCodeRegistry in RuntimeWiring, or possibly not use RuntimeWiring at all.

To make the inspector more generally usable we could allow it to take a map of registrations, independent of how or whether RuntimeWiring is used. A map is suffiicent for the internal implementation, in which case the existing method taking RuntimeWiring would become a simple delegate.