Arbitrary field interface.
This interface allows clients to store arbitrary fields for contacts in backends that support it.
This interface should be used for application-specific data, in which case the application should use the vCard approach to prefixing non -standard property names: `X-[APPLICATION NAME]-*’. Note that this is a global namespace, shared between all consumers of the backend’s data, so please namespace application-specific data with the application’s name.
This interface should not be used for more general-purpose data which could be better represented with a type-safe interface implemented in libfolks. It must not be used for data which is already represented with a type-safe interface in libfolks.
A good example of data which could be stored on this interface is an e-mail application’s setting of whether a content prefers to receive HTML or plaintext e-mail.
A good example of data which should not be stored on this interface is a contact’s anniversary. That should be added in a separate interface in libfolks.