feat(discovery): add cryostat-agent discovery plugin

[andrewazores]

Related to https://github.com/cryostatio/cryostat/issues/936

Just a draft for now, mostly meant to share what the setup looks like to add the new cryostat-agent to a Quarkus containerized application.

[andrewazores]

The agent JAR is also now published (automatically) as a package on the GitHub Packages Maven repository:

https://github.com/cryostatio/cryostat-agent/packages/1601815

But this requires a GitHub Personal Access Token to use - GitHub still doesn't allow unauthenticated package downloads. For now I'll stick with using jitpack in this draft for the convenience. Ideally we publish the agent JAR to some repository that allows anonymous downloads.

The latest commits use that GitHub Packages JAR, but this requires some setup for the end user in $HOME/.m2/settings.xml, which I have documented in the README. We can also set up the CI build on this repo to be able to pull from that package repository, so images would still be built by CI and published to quay.io after this is merged, including the officially CI-built upstream agent JAR in the image.

[ebaron]

Seems good overall. The GitHub Maven repository manual setup seems unfortunate, but I imagine this will integrate more smoothly with CI than needing to clone and mvn install the dependency like we do with -core.

[andrewazores]

Yea, I tried a few different things (now erased from the commit history) to try to get the CI build working within this PR, but I couldn't figure it out. Always a 401 response when the runner tries to download the agent .jar from the GH repository. But, I think consuming the JAR in this way with the dependency plugin is much more palatable than building the agent as a submodule.

Consumers of the upstream -agent .jar might be okay with this manual GH Packages setup step, otherwise maybe we need to look at publishing to ex. Maven Central when the agent is ready for primetime. For downstream builds there's already some system for publishing JARs, so that should be relatively straightforward.

[andrewazores]

Dropping this, since the Agent is now available on Maven Central as well as on quay.io as an init container (both unauthenticated), and we have other published examples of how to consume the Agent in an application container (ex. test-applications repository).