Add missing `as_sql()` for `__search` lookup

The (MySQL-only) `__search` lookup wasn't properly implemented using the new ORM lookup system. It's use would result into a runtime exception:

```python
 return node.as_sql(self, self.connection)
  File "/site-packages/django/db/models/sql/where.py", line 106, in as_sql
    sql, params = qn.compile(child)
  File "/site-packages/django/db/models/sql/compiler.py", line 80, in compile
    return node.as_sql(self, self.connection)
  File "/site-packages/django/db/models/sql/where.py", line 106, in as_sql
    sql, params = qn.compile(child)
  File "/site-packages/django/db/models/sql/compiler.py", line 80, in compile
    return node.as_sql(self, self.connection)
  File "/site-packages/django/db/models/lookups.py", line 150, in as_sql
    rhs_sql = self.get_rhs_op(connection, rhs_sql)
  File "/site-packages/django/db/models/lookups.py", line 154, in get_rhs_op
    return connection.operators[self.lookup_name] % rhs
KeyError: 'search'
```

This implementation uses the existing SQL template provided by `BaseDatabaseOperations.fulltext_search_sql(field_name)`.