Readonly & internal custom fields

[michaelbromley]

Is your feature request related to a problem? Please describe.
Sometimes you want to define a CustomField which is only used for internal business logic, and should not be exposed by the GraphQL API. Other times you want a CustomField which is exposed, but cannot be modified through the GraphQL API (only by means of direct manipulation i.e. in the code of a Plugin).

Describe the solution you'd like
There should be 2 additional options available on all CustomField types:

• readonly: boolean: This field will be exposed via the GraphQL APIs but will not extend the related input types.
• internal: boolean: This field will not be exposed at all via the GraphQL APIs.

In the Admin UI app, readonly fields will be displayed as a read-only control, whereas internal fields will not be shown at all.