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']
)

• [X] 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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