Do not rely on the Job's triggerSCM Settings for SVN

Hello,
First, I'd like to thank you for your plugin, it's pretty amazing.

I have an issue with the choice made to recover the SVN Authentification from the Job itself.
I can see [here](https://github.com/jenkinsci/last-changes-plugin/blob/cd15d8b6ef762726644a13a76aa376aab615097e/src/main/java/com/github/jenkins/lastchanges/LastChangesPublisher.java#L160) that you rely on the scmTigger settings to authentify into SVN.

The problem is that our workflow is not that simple.
We're using Jenkinsfile which are stored in a git repo. (Thus the SCM settings of the job are set on git)
During the pipeline, we manually checkout the sources from a Subversion repository with supplied Credentials.
Then I'm using the LastChanges plugin to review the changes of the svn repo (using the wonderful vcsDir parameter).

You guessed it :
`Problem creating svn auth provider
java.lang.ClassCastException: hudson.plugins.git.GitSCM cannot be cast to hudson.scm.SubversionSCM
at com.github.jenkins.lastchanges.LastChangesPublisher.perform(LastChangesPublisher.java:153)
at com.github.jenkins.lastchanges.pipeline.LastChangesPublisherScript.doPublish(LastChangesPublisherScript.java:32)`

Can you add an optional parameter to specify the creds of the repository in which LastChanges will go check it's stuff and build the ISVNAuthenticationProvider yourself ?
If this parameter is not provided, go back to the current behavior.

This could simplify #10,
And is quite dimilar to the #25

Thank you for your time