gafferpy API missing a basic Pair implementation

[tb06904]

The current gafferpy API has an implementation for a SeedPair that makes use of the class uk.gov.gchq.gaffer.commonutil.pair.Pair however, this only accepts a single EntitySeed for both the first and second argument of the pair. A generic Pair class is needed in gafferpy so that operations such as GetElementsBetweenSetsPairs can be used.

This was found under testing of the GetElementsBetweenSetsPairs operation as currently you cannot use this operation via gafferpy due to missing suitable class for the input to the operation. A class needs adding that can accept both a pair of objects or a pair of two lists and serialise them correctly into JSON.

For reference valid JSON for the GetElementsBetweenSetsPairs operation looks like:

{
    "class" : "GetElementsBetweenSetsPairs",
    "input" : {
        "class" : "Pair",
        "first" : {
            "Iterable": [
                {
                    "class" : "EntitySeed",
                    "vertex" : 1
                }
            ]
        },
        "second" : {
            "Iterable": [
                {
                    "class" : "EntitySeed",
                    "vertex" : 2
                },
                {
                    "class" : "EntitySeed",
                    "vertex" : 4
                }
            ]
        }
    }
}