Add CProverString.format function

This directly translates to a builtin function of CBMC string solver.

Author: romainbrenguier

src/main/java/org/cprover/CProverString.java

@@ -410,4 +410,15 @@ public static String ofCharArray(char value[], int offset, int count) {
         }
         return builder.toString();
     }
+
+    /**
+     * Format a string according to the given {@code format}.
+     * Unlike the java version, all arguments are given by strings, there is
+     * a fixed number of them and they should not be null.
+     */
+    public static String format(
+            String formatString, String arg0, String arg1, String arg2, String arg3,
+            String arg4, String arg5, String arg6, String arg7, String arg8, String arg9) {
+        return CProver.nondetWithoutNullForNotModelled();
+    }
 }