Add integration with RunConditionStep plugin

[mkroutikov]

Problem I am solving: skip Pipeline step if file already exists in Google Storage bucket.

The best solution I see so far is to have Google Storage Plugin define an extension point to RunConditionStep plugin and define this functionality, using syntax like this:

googleStorageFileExists fileName: 'gs://bucket/folder/file.xml', credentialId: 'mikes-gs-secret'

See here for hints how to extend the RunCondition: https://wiki.jenkins.io/display/JENKINS/Run+Condition+Example+Plugin

Thank you!