Fix compatibility with configuration cache

[melix]

This commit reworks the agent task actions so that they are compatible with the configuration cache. The problem was that when the configuration cache is enabled, it will try to serialize the task actions that we add, so that if the task is not up-to-date and that the configuration didn't change, it can replay the action.

The problem was that our task action used, as an input, a Provider<List<String>>, where those strings represented paths which do not exist when the action is serialized yet. In practice, we don't care, because those are implementation details of the task action: the directories will not exist before the task is executed, and not after, they only exist during task execution.

The configuration cache, however, wants to know about all inputs. We could have changed the action so that it uses a root directory instead of a list of directories as input, and make the computation of the session- directories during the task action: this would have worked because the only input would be the "output directory" that we pass as root. However, the task action is also used in another task which uses completely different inputs, which are not derived from a single directory.

At the same time, we don't need the Provider semantics in the task action: all we care about is to have a supplier which will generate a list of directories. Therefore, we changed the Provider type to a Supplier one: the lambda will be serialized, but the only thing it will capture is the outputDir provider which is fine, configuration-cache wise.

Now, let me take a pill.

Fixes #302

[melix]

Apparently this fixes storing in the configuration cache, but reading still fails with:

* What went wrong:
Could not load the value of field `__tasksToInstrumentPredicate__` of `org.graalvm.buildtools.gradle.dsl.agent.AgentOptions` bean found in field `capturedArgs` of `java.lang.invoke.SerializedLambda` bean found in field `transformer` of `org.gradle.api.internal.provider.TransformBackedProvider` bean found in field `left` of `org.gradle.api.internal.provider.OrElseProvider` bean found in field `provider` of `org.gradle.api.internal.provider.MappingProvider` bean found in field `__agentMode__` of `org.graalvm.buildtools.gradle.internal.AgentCommandLineProvider` bean found in field `jvmArgumentProviders` of `org.gradle.process.internal.DefaultJavaForkOptions` bean found in field `forkOptions` of task `:test` of type `org.gradle.api.tasks.testing.Test`.
> org.graalvm.buildtools.gradle.internal.DefaultGraalVmExtension$$Lambda$2609/0x00000008021065a8

[sdeleuze]

Not sure I am fully qualified to evaluate properly this PR, but the code changes LGTM. I will double check before the release it does not introduce regressions with Spring apps.