add optional unofficial releases watch list

[unitycoder]

To get alpha versions listed earlier, can search them with google api.
(sometimes they are available, like 6000.1.0a6 is now, before its listed in releases api)

• Add settings option for "fetch unofficial releases list" or so
• Fetch data from my commit https://github.com/unitycoder/UnofficialUnityReleasesWatcher/blob/main/unity-releases.md *not working so well now, maybe unity has cleaned these up?
• fetch data from github repo (gist RAW link changes per update..)
• Setup script to update that gist automatically (on my server)
• to make it more transparent, when user clicks install buttons, could show download url (can also validate that its unity3d domain)
• add 6000.0.x releases also? *not enough search credits.. so only 6000.2 for now
• add 6000.2.x releases also
• dont display releases that are already in the unity releases list https://services.api.unity.com/unity/editor/release/v1/releases?limit=20&offset=0
• should keep all found releases, until its available in unity releases api (sometimes they disappear from my list, since its not found with the fetch)
• server fetch is still bugged, displays old available versions too..
• Updates list shows duplicates, since server fails to clean old releases from github list

[unitycoder]

this is available in the build, so will improve more later..

[unitycoder]

• fails to download build from unofficial list "download exeURL= ()" *because previously cached unofficial versions are listed in cache (and without the directURL), so download button thinks its regular version and tries to search from unity Archive site
• null ref for unofficial full editor download exeURL = exeURL.Replace("UnityDownloadAssistant-" + version + ".exe", ...