Add the static data resource base class

Problem/Motivation

Every front end starts by fetching one bundle of site-wide data: menus, phone numbers, social links, schedules, counters. What goes into it is per site, which is why it is a convention rather than a fixed list, and why the current implementation is a long hand-written method that no other site can use.

Proposed resolution

• MyrestStaticDataResourceBase assembling the blocks named by the static_data convention, each block a small plugin or a service call.
• Menus rendered through a shared builder with depth and access handling, since the same output is needed inside other responses.
• A block whose convention is unfilled is absent from the response — no empty key.
• Cache tags per menu, per configuration object and per referenced entity, so that editing a phone number invalidates this response and nothing else.

Remaining tasks

Everything.