Allow multiple engine types per operator

[kyprifog]

Currently Each operator just supports one engine of each type. Would like to have multiple engines per type, for example:

metastores: [hive, s3]
execution: [spark, dask]

And ability to use multiple of them within an operator

[kyprifog]

The foundation for doing this was already done in previous commits. I am still looking for a good first candidate to finish out the implementation though.