Enable nullable references... and other stuff.

[bitbound]

Unfortunately, this is the ultimate "while you're in there" PR.

• Enabled nullable references on all projects and refactored to eliminate warnings.
• Extracted logic from FileLogger to IFileLogsManager.
• Made IFileLogsManager read all log files and return them as IAsyncEnumerable, so it's not loading everything into memory.
• Reversed implementation of basic authorization.
  • Re-implemented API keys using X-Api-Key header.
• Refactored ApiAuthorizationFilter and ExpiringTokenFilter.
• Made debugging use constant Org ID and device ID.
• Replaced concurrent download throttler with aspnetcore's new built-in rate limiting middleware.
• Moved entities to new namespace/folder.

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[bitbound]

This now reads all files instead of only today's. Returns IAsyncEnumerable so it doesn't have to load all files into memory at once.

[cmbankester]

Should we also add a cancellation token parameter here, with the EnumeratorCancellation attribute?

[bitbound]

Good call. I added it for all the methods in this class.

[cmbankester]

Should we also add a cancellation token parameter here, with the EnumeratorCancellation attribute?