Add missing $context parameter to XmlSitemapLinkStorageInterface::save()

Problem/Motivation

XmlSitemapLinkStorageInterface is outdated and no longer matches the methods of XmlSitemapLinkStorage:

• save() takes a second parameter array $context = [] in the implementation, which it passes to hook_xmlsitemap_link_alter(), hook_xmlsitemap_link_insert() and hook_xmlsitemap_link_update(). The interface declares only save(array $link).
• create() and save() return the sitemap link array, but the interface documents no @return.

Proposed resolution

Update the interface to match the implementation: add array $context = [] to save(), and document the return values of create() and save().

API changes

XmlSitemapLinkStorageInterface::save() gains an optional $context parameter. Other implementations of the interface must accept it.