[SCM-832]  maven-scm-provider-jgit should support SSH public key auth (#50)

* Support public key auth for SSH #SCM-832

Adds a TransportConfigCallback to all remote commands, which adds a public/private key based identity for repositories with ssh URLs if configured.

* Updated documentation for #SCM-832

* [SCM-832] Updated documentation

* [SCM-832] Added debug logging

maven-scm-provider-jgit now outputs the private key used when run as mvn -X
6 files changed