[IJ Plugin] Reconsider use Gradle's continuous mode for automatic codegen

[BoD]

Currently the automatic codegen feature works by invoking Gradle in continuous mode. While this was a handy way to implement this without having to replicate the file monitoring handled by Gradle, it has the disadvantage of keeping a daemon running always, in BUSY status (not available for other builds) - which consumes memory on the system.

Consider reverting to manually trigger the codegen task when GQL files are touched.

[BoD]

Even better if we can bypass Gradle completely and invoke the compiler directly. Work in #5516 should help for this!