..F....... [ZBX-8727] Fix formula validation in item configuration.
- Implement new decimal string validator CDecimalStringValidator. 
- Add new request parameter type T_ZBX_DBL_STR and use the new CDecimalStringValidator in it. 
- Change type of request parameter "formula" in item configuration to T_ZBX_DBL_STR. 
- Implement tests for CDecimalStringValidator.