Add (server-)id property to Scm model

The maven-release plugin introduces the IdentifiedScm which includes the id property, allowing the user to specify a server id for use with the lookup of credentials that can be set securely in the maven settings.xml.

This is a nice feature to have on the original Scm model as well and it will leverage some security issues that we have with multiple other plugins operating on the Scm model.

Currently, the user is required to specify the credentials either via the command line or directly in the plugin configuration in the pom, or externally, using third party tools.

And the Scm model should also implement the hashCode and equals() methods.

Of course this will require also changes to the existing Scm plugins so that they can make use of the new feature. Also, the maven-release plugin can be somewhat simplified by this change.

QUESTION: should this then still be the POM version 4.0.0 or is this then 4.1.0?