In Documentation
A hyperlink this cell points to, if any. This field is read-only. (To set it, use a =HYPERLINK
formula in the userEnteredValue.formulaValue field.)
You can't set multiple properties for an ExtendedValue object it only accepts single property to set. All properties point a value for Cell but only difference is those setters determines the type of cell value (bool, string, formula, number, error), so you can't set multiple value for Single Cell thats why you get the error( (oneof), oneof field 'value' is already set ).
Here is the new code, you can add hyperlink like this:
values.add(
new CellData()
.setUserEnteredValue(new ExtendedValue()
.setFormulaValue("=HYPERLINK(\"http://stackoverflow.com\",\"SO label\")")
);