Add protected typecasting setters to DataObject

## Add protected typecasting setters to DataObject

### Acceptance Criteria
1. `DataObject::setAsInteger($field, $value)` sets the given field as an integer.
1. `DataObject::setAsBoolean($field, $value)` sets the given field as a boolean.
1. `DataObject::setAsString($field, $value)` sets the given field as a string.

### Tasks
- Add methods.

### Additional Notes
- This code has already been written in a couple of projects -- might as well make it part of synapse base so that overwritten setters can utilize it.