Add support for authenticated requests when checking a release's build status

The org.apache.sling.cli.impl.ci.CIStatusValidator checks if a release's build status is valid, when verifying a release. Depending on the network from which the requests originate, GitHub might enforce its API rate limiting, leading to a failure in the validator - the success JSON key is missing from the response. On one hand, the validator's JSON parsing could be improved to not fail if this key is missing. However, if a request is authenticated (usually with a GitHub access token), the rate limiting is not enforced (or better said the limit is so high that it should not be reached).

This issue should implement both suggestions from above:

1. make the JSON parsing more resilient
2. optionally use a GitHub access token, if provided in the environment, to perform authenticated requests against the GitHub API