Stable machine id, e.g. "to" / "cc" / "bcc" / "required" / "optional".
Display label shown next to a contact chip, e.g. "To", "CC", "Required".
OptionaldefaultExactly one role per linkType should be marked default.
OptionalhiddenHidden roles are visible only to (a) the contact themselves and
(b) the user who added them. The API filters them out of every other
viewer's thread.contacts and thread.contactMeta. Use for BCC-style
semantics where other recipients must not see the hidden contact.
Declares one contact role for a connector's link type. See
LinkTypeConfig.contactRoles.