Support unknown/extra components and properties

[allenporter]

Is your feature request related to a problem? Please describe.
Add support for any unknown or extra components, and improve unknown property support.

Describe the solution you'd like
Any component that is not known should be added as an extra component, similar to how extra properties are handled. e.g. a repeated list of unparsed components.

Extra properties should move away from using the internal ParsedProperty objects, and instead should use some new model specifically for extra properties (it can have the same structure).

In addition to decoding, these also must be serialized.