Add "serverId" and "privateKey" SCM parameters

[kwin]

Separate abstract classes for read and write SCM operation

This closes #1384

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[michael-o]

What is the motivation here?

[kwin]

To align with what is possible in maven-scm-publish-plugin. Also the hardcoded default lookup mechanism of server ids derived from URLs is just too static (often credentials are shared among multiple hosts/ports).

Also the undocumented property

maven-release/maven-release-plugin/src/main/java/org/apache/maven/plugins/release/AbstractScmReleaseMojo.java

Line 171 in c8cd8f0

descriptor.setScmId(project.getProperties().getProperty("project.scm.id"));

is just too well hidden.