Add a function to fetch information about a table

Mostly so whether a field can contain NULL values can be determined.

guix-data-service/model/utils.scm

@@ -114,6 +114,21 @@
         '()
         lst))
 
+(define (table-schema conn table-name)
+  (map
+   (match-lambda
+     ((column_name data_type is_nullable)
+      (list column_name
+            data_type
+            (string=? is_nullable "YES"))))
+   (exec-query
+    conn
+    "
+SELECT column_name, data_type, is_nullable
+FROM information_schema.columns
+WHERE table_name = $1"
+    (list table-name))))
+
 (define* (insert-missing-data-and-return-all-ids
           conn
           table-name