Extend CustomFieldConfig by "config" property

[chladog]

Is your feature request related to a problem? Please describe.
Currently there is no property in CustomFieldConfig that would allow developer to pass along some data/config object that is needed for example by custom UI component - CustomFieldControl.

Describe the solution you'd like
New property config OR data.
Usually the configs are objects so I propose JSON type for this property, though string would work as well as we can stringify JSON.

Describe alternatives you've considered
In the meantime I pass data in "description field, which is not the best soltution (cannot have actual description, and it's localized string)

Additional context
E.g. I want to make a custom UI component for customField which needs "schema" and some "config" options.

[michaelbromley]

I would like to incorporate this into ideas discussed in #415, so this data object would live under a ui: { ... } property, since it pertains specifically to the Admin UI representation of the custom field.

[martijnvdbrug]

Completed in #415