Bug 23336: Add checkout API's

Bug 23336: Add checkout API's

This patch adds API's to allow for a checkout flow using the RESTful
API.

We add an availability endpoint to check an items current availability
status. The endpoint can be found at `/checkouts/availability` and is
a GET request that requires item_id and patron_id passed as parameters.

We return an availability object that includes blockers, confirms,
warnings and a confirmation token to be used for checkout.

We also add a corresponding checkout method to the `/checkouts` endpoint.
The method accepts a POST request with checkout details including item_id
, patron_id and the confirmation token in the body.

Future work: We should properly migrate CanBookBeIssued into Koha::* and
use that here instead of refering to C4::Circulation.

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>