Create a custom field scoped to a specific contract type. The created field will have appliesTo: "contracts" and will be linked to the specified contract type.

Required Fields:

• name - Internal field name (lowercase with underscores)
• label - Display label
• fieldType - One of: text, number, date, dropdown, multi_select, url

Optional Fields:

• description - Help text
• required - Whether the field is required (default: false)
• defaultValue - Default value for new contracts
• choices - Required for dropdown and multi_select types