Support mapping of entry data to database columns

[ryanmitchell]

This is a WIP.

This PR makes it possible to store the contents of data handles in alternative columns in your database, eg you may want to store my_key in a column called my_column.

The benefit of this is it allows you to index that column separately which can speed up database operations.

Mapping can be added by adding a hook on the eloquent entry class, and updating the array $payload. This array is a mapping of data handles to database columns (note that only root-level data keys are supported).

        Entry::hook('data-column-mappings', function (array $payload, $next) {
            $payload = array_merge($payload, ['my_key' => 'my_column']);

			return $next($payload);
        });

Closes https://github.com/statamic/eloquent-driver/issues/272