Support materialization hook

[natemcmaster]

Add a hook for customizing the ValueBuffer => ClrType materialization step.

Just one usage scenario among many possibilities:
Materializing an Annotatable. I want to put shadow properties into annotations.

[rowanmiller]

Think this is a dupe of #240

[rowanmiller]

BTW I think you don't actually want shadow properties right... you want a property that is stored in the CLR type but to get/set the value you don't use a property getter/setter.

[natemcmaster]

Yes can be solved by #240. This could also be solved by #2968. I was aware of these issues when filing, but still think a materialization extension point could be useful.

[natemcmaster]

#2968 (comment)