Add credentials via environment variable.

[akshaal]

This PR enables the ability to pass additional credentials to Dependabot through the generic-update-script.rb. This is needed to be able to use the script in a self-hosted environment with maven, where Dependabot needs to access one or more private repositories without having to specify the repositories in the pom.xml file of each maven project within the environment. As the 'credentials' parameter is not specific to the maven parser/fetcher, it will likely be useful for other package managers as well.