items API responds with available dates (hardcoded for now)

[kenoir]

When requesting an item currently we work out the available dates in the wellcomecollection.org codebase (https://github.com/wellcomecollection/wellcomecollection.org/blob/main/content/webapp/components/ItemRequestModal/RequestDialog.tsx#L143).

In order to sustainably provide availability dates when items can be requested from offsite locations we should move to incorporate these into the items API response, optionally available with a suitable request parameter.

This is to support the offsite-requesting work where we will need to consider items requested with differing date constraints for each offsite-location.

[kenoir]

We expect this may require querying data on opening times from prismic (or perhaps proxied through the content-api).