Add a mirror field

Adds a field to the board that mirrors a field of another board. The new field takes the target field's type (and, for select types, a copy of its option list), is named Mirror <target field name> unless you supply name, and is read-only unless you set readOnly: false.

This only adds the field - the same as the Aptly app. A card shows a mirrored value once it is linked to a card on the target board (for example through a related-cards field) and is next saved; existing cards are not backfilled when the field is added.

Requires update permission and the key's "Edit Boards" access (off by default). A key's board allowlist applies to both boards: the target board must be one the key can access and must be API-enabled. The target field must be one of string, text, rich-text, address, email, url, datetime-local, date, number, percent, stageselect, singleselect, multiselect, tags, boolean, person, persons, relatedAptlets, files, file, money, user or lockAlias, and can't itself be a mirror field. Mirroring the same target field twice on one board returns 409.