SKETCH: add step for getting properties

[eaaltonen]

Add a groovy script step to fetch custom properties from Artifactory.

NOTE: this is functional, but not suitable to be integrated directly. Some of the changes should be made to https://github.com/jfrog/build-info instead. This PR shows how to serve our use case of accessing our custom metadata and is here to solicit discussion on the API.

This provides Artifactory users with access to their additional metadata associated with artifacts stored in Artifactory, and allows pipelines to make decisions based on that metadata.

This is called roughly as:

artifactory = newArtifactoryServer( ARGS )
def props = artifactoryGetProps(
    server: artifactory,
    relativePath: "PATH/TO/ARTIFACT"
    propertyKeys: ['PROPA', 'PROPB']
)

• This pull request is created in the jfrog/jenkins-artifactory-plugin repository.
• All tests passed. If this feature is not already covered by the tests, I added new tests.

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[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[eaaltonen]

I have read the CLA Document and I hereby sign the CLA

[eyalbe4]

@eaaltonen,
Thank you for your contribution. We are not promoting new features for this plugin. We strongly recommend users switch to the newer JFrog Jenkins Plugin, which offers significantly more features and functionality than this plugin, including the new functionality proposed by this PR. Both plugins can be used side-by-side within the same jobs.

[eaaltonen]

@eyalbe4: there is an issue that is preventing us from switching to the JFrog Jenkins plugin at this point. I've opened an issue via another channel.

If I split the parts from this PR that belong to build-info to a separate PR, would you be open to accepting them while the other issue is resolved?