Create SQL-based data node in demo-project

[MehdiNV]

Description

PR #1564 adds a 'preview' mechanism for (some) data nodes, allowing users to glimpse the origin of that data (e.g. this node was generated via SQL; what was that particular SQL code?).

This mechanism is only visible for data nodes w/ no files, and useful when you'd like to see the origin of a node who has no file.

There is no example of this though to show to users; aim of this PR is to add a dummy node so we can exhibit the new feature.

Context

We've added a new feature, but it's only visible if the users specifically use a SQLQueryDataset node. If they don't, they may not even understand that the feature exists.

Point is to add a SQLQueryDataset data node in demo-project, to showcase the feature.

Possible Implementation

A partial implementation can be in this commit - PR #1564 removed this change later on though to reduce scope (and ship out faster, as this change can just be a separate issue instead of one giant large PR).

To complete this implementation, you may need to setup a SQLAlchemy connection (otherwise pipelines may fail, as they expect SQLQueryDataset to be connected to a SQL database) or some mock dummy SQL DB (SQLite?).

[tynandebold]

How much work would it be to add this to your PR? #1564