Make GenerateApolloSources a Gradle Incremental task.

[martinbonnin]

To speedup the codegen step, we could use incremental tasks (not to be confused with incremental builds that we already support) and only recompile the parts that changed.

A first step could be to introduce a first parseSchema task that caches the parsing and validation of the schema and extra.graphqls. Ultimately though, recompiling based on query changes is certainly what is going to bring the most improvements