adding mssql support

[vandsh]

making modifications to data-service to hopefully support MSSQL, not sure what else is needed on the front end to accomodate

[manisandro]

Thanks for the PR.

As far as the data-service is concerned, I'd suggest transforming dataset_features-provider into a base class with the common logic (if any), and implement it for dataset_features_provider_postgres and dataset_features_provider. Then, you could dynamically load the dataset_features_provider based on the dataset backend.

This would allow splitting loading the mssql bits only in a Dockerfile.mssql variant, and not directly in the base variant, since not all users necessarily want the proprietary mssql driver in the data-service base image.

As far as other changes, you'll also need to extend the qwc-config-generator to handle mssql datasources, not just postgresql.

[vandsh]

please also see qwc-services/qwc-config-generator#99

[manisandro]

This seems unused?

[manisandro]

Looks like this file is now also unused?