Showcase how to integrate DB-specific functions (#201)

Augment `AdminService.Author` by 2 fields `age` and `lifetime` whose
values are computed by SQLite and HANA-specific functions.

Don't do this in bookshop directly, to keep it simple.  Instead, use
the fiori module, which also allows to integrate the fields in the UI.

Co-authored-by: Daniel <daniel.hutzel@sap.com>

fiori/db/index.cds

@@ -0,0 +1,8 @@
+using { sap.capire.bookshop } from '@capire/bookshop';
+
+// Forward-declare calculated fields to be filled in database-specific ways
+// TODO find a better way to have 'default' fields that still can be overwritten.
+extend bookshop.Authors with {
+	virtual age: Integer;
+	virtual lifetime: String;
+}