Support testing SNAPSHOTs that have a deterministic git revision

[jtnord]

If a SNAPSHOT build is in the override war and it was built with a version of maven-hpi-plugin 3.42 or higher (jenkinsci/maven-hpi-plugin#436) then we have a concrete git sha we can use for checkout of the plugin.

So this relaxes the enforcement of not building non releases by allowing to test a SNAPSHOT also so long as we have a git commit for it.

in the case the commit is HEAD (which is set by the legacy plugin extractors) then we will refuse to test the plugin the same as we do today.

Testing done

• I deployed a unique snapshot of a multi-module project (version number made unique by changing it to 2.25-JTN-SNAPSHOT
• The deployed version was then used in a custom war (2.25-JTN-20230512.152321-1)
• The war was then run through CloudBees' PCT setup

I manually inspected the resulting logs and ensured that:

• Each plugin from the multi-module project was checked out and had the PCT run against it correctly
• plugins that dependend on any of the plugins deployed in step 1 where correctly updated by maven-hpi-plugin to use the SNAPSHOT version of the dependency

e.g.

[2023-05-12T21:13:02.916Z] [INFO] After resolving,
 updates: {com.cloudbees.jenkins.plugins:cloudbees-casc-items-controller=2.25-JTN-20230512.152321-1, ....

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[jtnord]

in order to test other plugins against snapshots we need jenkinsci/maven-hpi-plugin#478 released in 3.44.