Content Downloading rework.

[LossyDragon]

Description

This PR completely redoes the ContentDownloader feature for JavaSteam.

Notables changes:

• Separate feature as a child dependency (javasteam-depotdownloader)
• Aligns with SteamKit's DepotDownloader in terms of features.
• SampleDownloadApp sample reworked to show how to use DepotDownloader
• Bump to 1.8.0-SNAPSHOT

Drafting this for now to let some Android related projects try it out and report any issues before finalizing.

Checklist

• [x] Code compiles correctly
• [x] All tests passing
• [x] Samples run successfully
• [x] Extended the README / documentation, if necessary