Add http headers authentication support for private repositories

**Problem:**
There are private repositories such as the Gitlab ones ([Gitlab Repository Authentication](https://docs.gitlab.com/ee/user/packages/maven_repository/index.html#authenticate-to-the-package-registry-with-maven)) which require and support only HTTP headers authentication.

rules_jvm_external is only supporting HTTP basic authentication. ([link](https://github.com/bazelbuild/rules_jvm_external/blob/master/README.md#usage))
> Credentials for private repositories can also be specified directly as username:password@, using a property file or environment variables. See the Coursier documentation for more information.

I've found [that #80](https://github.com/bazelbuild/rules_jvm_external/issues/80) issue which looks similar(by its misleading title) but it seems that it was raised to tackle another problem: the property file and env variable with Coursier.

Start with the authentication section in README and the Coursier documentation referenced there; compare the current basic-authentication configuration with GitLab's linked header-authentication requirements. Review issue #80 to distinguish this request from earlier Coursier property-file and environment-variable work. Done means private repositories requiring HTTP-header authentication can be resolved.

Written by the indexing model from the issue text.