Add 'readonly' attribute to fields in schema

This is needed by Afform and SearchKit to know whether a field is appropriate to show to the user.

The goal here is to add 'readonly' to all fields that should not be presented as editable on user-facing forms.

Examples of fields that should be considered "readonly"

• Contact type (theoretically could be changed, but only under special circumstances and there's an extension for that)
• ID fields
• Contact Hash
• Custom fields marked "is_view"
• FK fields that are not meant to change, e.g. Email.contact_id or Participant.contact_id. (in rare circumstances this field gets updated e.g. during deduping, but it's still not something to present on a typical user-facing form, so I'd consider them 'readonly').
• Any auto-increment fields or autopopulated fields e.g. 'creation_date'.
• Any other fields that are typically written to programatically and not shown on forms.