feature: Have a way to collect untyped remaining fields.

[burdiyan]

Is your feature request related to a problem? Please describe.
Sometimes it's useful to have partially defined types for the data, but still being able to collect the remaining fields in more of a dynamic way as a map[string]any or map[any]any.

Describe the solution you'd like
Introduce a new struct field tag, similar to the remain tag in the popular mapstructure library. The type of a field with such a tag should be a map, into which those values that are not defined on the concrete type could be collected for later access.

Describe alternatives you've considered
Having a named type for a map, like type UnknownFields map[string]any which when embedded into a struct would collect the remainder fields.

[fxamacker]

Thanks for opening this issue! This sounds useful and I will need to look into it.