Add endpoint to populate combobox/select

[mataca9]

Add endpoint to populate a combobox

• Extend function selection on JSON definition with endpoint
• Create service based on components name example: PersonalInformationService
• Create service function based on form item key example: cities to getCitiesOptions()
• Create service call inside function
• Populate a variable with data
• Create this service only if some combobox property has URL

[mataca9]

Depends on: https://github.com/ilegra/form-schematic/issues/16