GUACAMOLE-2139: Automate retrieval of dependency license information.

[mike-jumper]

These changes update our existing license information and tooling such that they can be automatically pulled going forward. The output LICENSE and NOTICE is still the same - there's just no longer any need to create and manually format README files for each dependency, to manually update all license info when dependencies change, etc.

Instead of a README file, metadata is embedded directly in headers of downloaded copies of license information, bracketed by --- BEGIN LICENSE FILE --- and --- END LICENSE FILE --- lines. A Source header along with variable substitution allows the copy of license information to be pulled automatically when the version changes.

Manual license information updates are still needed if:

• New dependencies are introduced
• A dependency does not consistently host a copy of their license (more common for older libraries)
• Source for a dependency is not publicly available (BouncyCastle FIPS distributions) or is only available in archives (some extremely old Java APIs)

Opening as a draft while I update docs and clean things up a bit.