Rework app cast handling; add JSON compatibility; add AppCastItem.Channel; built-in channel-based app cast filter implementation

[Deadpikle]

Reworks app cast handling by separating out some logic of things, adjust return parameters, and more. Also adds JSON app cast handling and ChannelAppCastFilter class.

• Consider further changes to DownloadAndParse (split into separate download and parse functions?), GetAvailableUpdates (consider rename or different FilteredAppCastItems property or similar for greater clarity on what is happening)
• Consider removing AppCastHandler entirely and breaking out its functionality to different objects that are more specific (filtering to a filter object, etc.)
• Decide whether AppCastHandler should hold an actual AppCast object and, if not, who should. (What state is held where, in other words?)
  • Probably will make main SparkleUpdater keep a most recent parsed cached version for safe keeping but otherwise not hang onto it
• Use async methods from app cast generators, IAppCastDataDownloader.DownloadAndGetAppCastDataAsync
• Add JSON app cast handling (implements IAppCastGenerator)
• Add unit tests for JSON app cast handling to core lib
• Add unit tests for async app cast generator methods
• Add tests that items are sorted after Generator parses them out
• Add unit tests for JSON app cast handling to app cast generator
• Add unit tests for changing signature enclosure title in XML output to app cast maker, etc.
• Add tests for ChannelAppCastFilter
• Update output JSON names for AppCastItem, AppCast, etc.
• Update UI projects, samples as needed
• Finish adjustments to XML serialization/parsing to match normal Sparkle output (Enhance NetSparkleUpdater app cast compatibility with Sparkle app casts #275), including applicable changes to app cast generator (this should be fairly easy to do now with another option or two on both the SparkleUpdater end and the app cast generator end -- basically, let the user determine what the title for signature is in xml app casts)
• Support <sparkle:channel> for channels outside of Version property (incl. tests, app cast gen, usage in ChannelAppCastFilter)
• Consider making a bunch of things async for easier use for potentially long operations
• Consider fixing UI-created issue where download window and update available window show wrong data (don't want to break anyone building from source, I guess...although it does bleed into the UI refactor a bit...)
• Fix any remaining warnings, documentation issues
• Warn users somewhere about this change in .NET 8 (adds git commit to assembly version) which affects this and is confusing for end users: https://learn.microsoft.com/en-us/dotnet/core/compatibility/sdk/8.0/source-link

Closes #275
Closes #598
Closes #599