[ios] Convert UTF-8 encoded NSData to NSString

With Swift 5, you can use String's init(data:encoding:) initializer in order to convert a Data instance into a String instance using UTF-8. init(data:encoding:) has the following declaration:

init?(data: Data, encoding: String.Encoding)

Returns a String initialized by converting given data into Unicode characters using a given encoding.

The following Playground code shows how to use it:

import Foundation

let json = """
{
"firstName" : "John",
"lastName" : "Doe"
}
"""

let data = json.data(using: String.Encoding.utf8)!

let optionalString = String(data: data, encoding: String.Encoding.utf8)
print(String(describing: optionalString))

/*
 prints:
 Optional("{\n\"firstName\" : \"John\",\n\"lastName\" : \"Doe\"\n}")
*/