feature: pass context to CustomUnmarshaller

[fortuna]

Describe the solution you'd like
Please provide a CustomUnmarshallerContext interface that takes the context. Currently there's no way to receive the context with a CustomMarshaller.

I need to implement an unmarshaller that unmarshals "named" objects by looking them up on a map that is kept in the context.

Describe alternatives you've considered
InterfaceUnmarshalerContext and BytesUnmarshalerContext don't meet my need, since they require adding behavior to the type being unmarshalled, and also ties the type to this library, but I want my config types to be library-agnostic.