feat: improve consistency of many arg with nested schema

[jafournier]

For nested field, we can get some inconsistencies between field.many and field.schema.many.

Here is a proposal to handle inconsistency edge cases (see tests).

PS: Such inconsistencies have some effects on automatic swagger generation in flask_appbuilder (and thus in projects using it, as superset).
This will solve apache/superset#33884.