Add behaviour control for missing fields

Add the ability to allow missing fields

By default this maintains compatibility with the original implementation which:
* Errors if a struct field is missing from the result set.

Creating a custom client allows the configuration of how:
* Missing fields are handled (AllowMissingFields)

This is useful as it allows the full struct definition to be used with custom queries that only return a partial set of information which they require, instead of having to create an alternative stuct only those fields.