Add getter functions for wasm Lox credential attributes

Adding functionality to allow the Lox client to extract certain attributes from a user's Lox credentials in order to display the information to a user to meet the designs described here: tor-browser#42036. Addresses part of #41 in that it will allow a user who has issued an invitation to know when their invite will expire. It will also allow an invitee to know when their invitation expires. Determining whether or not an invite has already been claimed is a more involved process (see #41) that will not be prioritized for alpha.