Invoke the Apollo compiler directly, when possible

[BoD]

• Retrieve the "project model" (list of Apollo Services and their configuration) by invoking the Gradle task introduced here that dumps JSON files, and reading them. If this fails (e.g. the project is using an older version of the library), we fall back to the current mechanism of retrieving the Gradle tooling model
• Codegen: no longer run the codegen task in continuous mode (addressing [IJ Plugin] Reconsider use Gradle's continuous mode for automatic codegen #36). Instead, run the Apollo Compiler directly (or via the codegen task if in fallback mode) whenever a GraphQL file is touched

Missing:

• Pass more project information, for telemetry (e.g. versions of libraries, options used, etc.)
• Support cases where Apollo compiler plugins are used