Implementation of OrderByRaw method on Content Queries

[mbs-arran-baker]

It would be useful to have an implementation of the Laravel Query Builder 'orderByRaw' method on Statamic Content Queries. The current ordering methods are limited when trying to apply a custom order to query results.

For example being able to apply a defined order based on the returned results id:

$query->orderByRaw("FIELD(id, [2, 4, 7, 3, 9])")