Adds the UpdateManifest site key and associated client.  This required
some additional features in the existing update machinery.

Each "version" can now (optionally) have its own download URL.

Mod Page objects can now specify that subkey matching (for that page)
should be "strict".  A strict subkey match does not fall back to matching
with no subkey if a subkey was provided but produced no versions.  It also
strips the leading '@' from the subkey.

IModDownload objects are now responsible for deciding whether a subkey
matches or not.  The default behavior is unchanged, but this allows
different mod sites to have different rules for subkey matching (which
the UpdateManifest mod site uses to force exact matches).

UpdateKey parsing now allows multiple : and @ inside the update key,
splitting on the first occurence of each

Subkey matching is moved into IModDownload / GenericModDownload, in
preparation for some Mod Sites using something less error-prone than
substring matching.

The migrated package didn't work consistently in VIsual Studio, so this suppresses nullable annotations in .NET Standard instead.

This adds `#nullable disable` to all existing code (except where null is impossible like enum files), so it can be migrated incrementally.

This simplifies the logic for individual clients, centralises common logic, and prepares for upcoming features.