Add string comparison operations to the IF() function

[Nick-S-2018]

Proposal:

As of now, IF() supports only the '=' comparison for string values. We need to add support for the '<', '>', '<=' , '>=' operations too.

Checklist:

^{To be completed by the assignee. Check off tasks that have been completed or are not applicable.}

• Implementation completed
• Tests developed
• Documentation updated
• Documentation reviewed
• Changelog updated
• OpenAPI YAML updated and issue created to rebuild clients

[Nick-S-2018]

Related to #832

[tomatolog]

added at 3ab7ee8 string comparision operators in the select list expressions. Now expressions with string attributes could be compared with <,>,<=,>=,!= in the select list and other expressions such

select if (str_attr<'that', -2, 100) as cnd1 from idx ...