[swift5] Add property `x-null-encodable` extension for full control over encoding value/nil or nothing

Created by: jarrodparkes

Attempts to solve #11033 (closed)

This PR adds a new property vendor extension x-null-encodable for use with the swift5 generator. If set to true, then the generated properties will use the NullEncodable<Wrapped> type:

public enum NullEncodable<Wrapped> {
    case encodeNothing
    case encodeNull
    case encodeValue(Wrapped)
}

This type gives the client full control of whether to encode a value/null -OR- encode nothing.

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