Support username/password auth pulled from .m2/settings.xml

[cgruber]

In issue bazelbuild/bazel#7443 it is mentioned that the newer jvm_maven_import_external does not support user/password authentication in fetching using definitions in .m2/settings.xml.

We should look into this as an option, for cases where non-ssh authentication to non-public repositories is required (e.g., https with username/password).