OPENNLP-1659 - Enhancements for DownloadUtil

[rzo1]

For all changes:

• Is there a JIRA ticket associated with this PR? Is it referenced
  in the commit message?

• Does your PR title start with OPENNLP-XXXX where XXXX is the JIRA number you are trying to resolve? Pay particular attention to the hyphen "-" character.

• Has your PR been rebased against the latest commit within the target branch (typically main)?

• Is your initial contribution a single, squashed commit?

For code changes:

• Have you ensured that the full suite of tests is executed via mvn clean install at the root opennlp folder?
• Have you written or updated unit tests to verify your changes?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE file, including the main LICENSE file in opennlp folder?
• If applicable, have you updated the NOTICE file, including the main NOTICE file found in opennlp folder?

For documentation related changes:

• Have you ensured that format looks appropriate for the output in which it is rendered?

Note:

• Make the BASE_URL and MODEL_URI_PATH configurable via System properties in order to be backwards compatible if models are removed from the release area of OpenNLP / ASF CDN
• Make the OPENNLP_DOWNLOAD_HOME configurable, i.e. for tests in CI environments. This can also be useful for OpenNLP devs to avoid cleaning models in their user home.
• Replace hard coded file separators for multi OS support
• Log a DEBUG message if the model file already exists and the download is skipped.
• Adds a test case to verify that models are not downloaded twice (if they exist) by testing for the related LOG output using LogCaptor.
• Replaces slf4j-simple with logback (from LogCaptor) for further testing of specific output