Field.try_merge should have optional metadata merge

[ion-elgreco]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Merging fields is useful but the implicit merge of metadata is something that should be opt-in. At the moment we have to undo this to get the required effect.

[tustvold]

I wonder if we really just need to provide a visitor API to allow people to define what schema merging logic they desire... We can then implement try_merge using this, and if people want something different they can implement their own visitor