sql: introduce function default values

This patch replaces the SQL mechanism for setting expression default
values with the BOX mechanism.

Closes #8793

@TarantoolBot document
Title: field default value in SQL

SQL now uses the default mechanism for BOX fields. The main difference
visible to the user is that now all NULL values explicitly inserted into
a field with a default value will be replaced with a default value,
which is different from the original SQL behavior. Additionally, default
values are now inserted instead of NULL after the SQL BEFORE INSERT
trigger but before the BOX BEFORE INSERT trigger. Previously,
SQL default values were inserted before the SQL BEFORE INSERT trigger.