Add Media Player macros

[agnessnowplow]

Describe the feature

1. New macro to be added to handle listagg/string_agg function differences across the supported adapters within the Media Player model to remove unnecessary code complexity.
2. The get_optional_fields macro to be altered so that the user can decide whether the field aliases are added or not - this is handy for the Media Player package when the columns extracted from BigQuery context fields need to be renamed immediately or the syntax does not need them there at all. The altered version is currently within the model itself so that should be removed.