Create and read custom fields on Pipedrive deals via the API

Steps

1. Create a custom field by POSTing to /v1/dealFields with a JSON body containing name, field_type (e.g., text, date, enum, monetary), and for enum types an options array
2. The response returns a key (a hash string) that is the API name for that custom field — store this for use in deal read/write operations
3. Write the custom field on a deal by including the field key and value in a PATCH to /v1/deals/{id}
4. Read custom field values from a deal via GET /v1/deals/{id} — custom fields appear in the response alongside standard fields, keyed by their hash key
5. List all custom field definitions with GET /v1/dealFields to map hash keys back to human-readable names