Provide `DROP TABLE IF EXISTS` wrapper

[vadz]

It would be nice to have a way to drop a table which may not exist other than

    try {
        sql << "drop table soci_test";
    }
    catch (...) {}

which is used now in SOCI's own tests. Of course, most databases support DROP TABLE IF EXISTS nowadays, but some of them don't and I wonder if we can provide workarounds for them.

1. DB2: no if exists, can do something like SELECT * FROM information_schema.tables WHERE TABLE_SCHEMA = 'MY_SCHEMA' AND TABLE_NAME = 'MY_TABLE'; according to this SO answer.
2. Firebird: doesn't seem to have support for if exists, does have RECREATE TABLE.
3. MySQL: supports if exists.
4. Oracle: only supports if exists since 23c, this SO answer shows how to ignore exception from DROP TABLE.
5. PostgreSQL: supports if exists.
6. SQLite: supports if exists.
7. SQL Server (via ODBC): can use select object_id('table_name','u') to check if it exists.

Thinking more about this, maybe what we really want is a way to execute an SQL statement without throwing an exception on error, so that we could use it to ignore the error from drop table without throwing (and catching).