Automatically set recommended JVM flags in Gradle builds

[Sanne]

Description

The work on #49920 introduced a ModuleOpenBuildItem, which dynamically applies Java module configurations to the JVM launched for dev-mode or integration tests.
This needs either our agent to be attached, or some JVM flags to allow us to take control of such aspects; when working on #49920 we made sure that a Maven build would automatically set such flags; when they are not, things will still work but there will be actionable warnings logged, so any Gradle user won't have problems either but they might need to add some more JVM flags to their build scripts.

It would be ideal to adapt our Gradle plugins to setup such flags automatically, so that users don't need to bother with any action at all.

Implementation ideas

See a3387cb

[quarkus-bot]

/cc @glefloch, @quarkusio/devtools