`is_multiple` vs custom fields

Right now I don’t get multiple values in a custom field that does not inherit from the List field because is_multiple only checks for this.

And for some cases rewriting the field to be nested in an actual List field is not feasible, for example a SQLAlchemyModelList-like field where sending individual queries would be inefficient compared to one query with an SQL IN filter.

I propose adding a MultiValueField which would simply inherit from Field and do nothing itself, and instead of checking for isinstance List check for that one in is_multiple. List would then of course inherit from that instead of Field.

Alternatively, check for something like isinstance(field, List) or getattr(field, 'is_multiple', False).

I can send a PR if there’s interest in this.

[lafrech]

I did a few CHANGELOG updates. I don’t think it was worth a PR so I pushed to dev.

[lafrech]

Also we might want to add a paragraph in the upgrading section of the docs.

https://webargs.readthedocs.io/en/latest/upgrading.html