Rework observations classes to better interact with Json source generated serialization

[rabuckley]

This PR

• Makes it possible for external assemblies to use source generated JSON serialization on observation types, without requiring us to export a JsonSerializerContext with access to internals.
• Removes the mess of JsonContructors, largely replacing with required init properties.
• Removes JsonIgnore DateTimeOffsets with linked long timestamps using the unix time JSON converter and truncating to milliseconds on init (for Assert.Equivalent to pass in tests).
• Extends tests of JsonSerialization, adding coverage all the way up to SnapshotSet.