Dynamically apply Add-Opens directives from MANIFEST.MF

[basil]

Currently, people testing with Java 17 must add an awkward -DargLine= directive with the Add-Opens modules from the core version they are testing. This PR applies a trick to dynamically set this based on the values in MANIFEST.MF.

With this in place, anyone with a sufficiently recent core (2.341+), test harness (1721.v385389722736+), and maven-hpi-plugin (this PR) should be able to effortlessly do Java 17 testing and indeed enable Java 17 in their Jenkinsfile. Once this is merged, I will try to do a release and wrap this and the test harness change into a new plugin POM release so that the requirement just becomes "core 2.341+ and plugin POM 4.40+".

Tested with workflow-job and Java 17:

$ mvn clean verify -Dtest=org.jenkinsci.plugins.workflow.job.WorkflowJobTest,org.jenkinsci.plugins.workflow.job.WorkflowRunTest -Djenkins.version=2.341 -Djenkins-test-harness.version=1723.vcd938b_e66072 -Dhpi-plugin.version=3.27-SNAPSHOT -Denforcer.skip
[…]
[INFO] --- maven-hpi-plugin:3.27-SNAPSHOT:initialize (default-initialize) @ workflow-job ---
[INFO] Setting argLine to -Xms768M -Xmx768M -XX:+HeapDumpOnOutOfMemoryError -XX:+TieredCompilation -XX:TieredStopAtLevel=1 --add-opens java.base/java.lang=ALL-UNNAMED --add-opens java.base/java.io=ALL-UNNAMED --add-opens java.base/java.util=ALL-UNNAMED
[…]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  02:14 min
[INFO] Finished at: 2022-03-30T11:21:40-07:00
[INFO] ------------------------------------------------------------------------
$

[jglick]

Nice. BTW how does this sort of thing get integration-tested? Does running existing plugin-pom ITs using Java 17 suffice?

[basil]

Nice. BTW how does this sort of thing get integration-tested? Does running existing plugin-pom ITs using Java 17 suffice?

I will write some tests once all these changes are merged and dependencies are updated. Right now there are too many dependencies on unmerged/unreleased PRs to be able to write tests easily.