Data validations and get data for dropdowns from an API

On a HTML form that gets fields and data from an API, I need to add some validations and get the data to the dropdown list from an API. More exactly:

a) Validations: Along the data that it is currently being sent to the API, we need to send which data type, field type and validation has every field. So the front-end can validate that the data the user has introduced is correct. By now, we need basic data types: string, integer/decimal, date. And basic field types: simple input text, textarea, date picker and dropdown (and show the field with these types, now it's always shown with a simple input text). Also we need basic validations: if the field is mandatory, range validation for integer/decimal/date types, and max length for string fields. The message that must be shown is also sent by the API. The validation must be done when fields loses focus (showing the message in a non-obtrusive way) and on submit.

b) If the field type is dropdown list, then the data in that list must be taken from a URI (rest API call), that will be sent alongside with all the data. This API will return a simple JSON key-value list.

This project does not include modify the API, only the front-end development on the HTML/JQuery form.