Bug 30979: Add public endpoints

Bug 30979: Add public endpoints

This patch expands the checkouts endpoints to allow for a public workflow.

We add the availability endpoint under `/public/checkouts/availability`
and restrict the information we send back to only those fields a public
user should be allowed to see.

We also add a new checkout endpoint at `/patrons/{patron_id}/checkouts`
that allows for users to checkout to themselves and accepts the same POST
request with checkout details including item_id and a confirmation token
in the body that the staff client endpoints accept.

Signed-off-by: Silvia Meakins <smeakins@eso.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>