Rework Gradle build

This commit introduces a _convention plugin_ for RIFE2 support.
It provides a number of advantages:

- the build logic is clearly separated from the build script,
which now only contains user-specific configuration, like the
framework version or how to configure test logging
- it fixes a number of issues like hardcoded dependencies
(`dependsOn` is in general a mistake)
- it removes the need to resolve the agent path eagerly
- it makes the agent configurable
- it clearly separates the user classpath from the RIFE
classpath (both for precompiling templates and for the agent)
- template compilation is cached
- it avoids the abuse of `src/main/resources` to put
templates and uses a dedicated directory instead, which
removes the need for exclusions

In addition, this should make it relatively straightforward
to convert the convention plugin into a proper RIFE2 plugin,
by extracting the code in `build-logic` into its own
repository then publishing to the Gradle plugin portal.

gradle.properties

@@ -0,0 +1,2 @@
+org.gradle.parallel=true
+org.gradle.caching=true