added buildUrl property for overriding the url used for the build / run

[marchaos]

This pull request allows overriding the url used for the build / run. This is useful in pipeline builds where you might notify bitbucket about a particular stage, where the url should go directly to the logs of that stage, rather than the top level pipeline. A new prop buildUrl allows you to override what is used, otherwise the default url that comes from DisplayURLProvider is used.

See feature request for more info - https://github.com/jenkinsci/stashnotifier-plugin/issues/302

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