[HDX-1848] Dashboards: Allow querying arbitrary tables.

[Defman]

You are currently unable visualize data outside of logs, traces, metrics and sessions tables. Support building and querying of arbitrary tables and thereby build custom visualization.

_HDX-1848

[MikeShi42]

I do agree we want to support custom/arbitrary visualizations (though mostly focused on o11y workflows) - something we want is to be able to fully define a SQL-based chart and everything just work. Though we also have the concept of sources & connections, where sources currently fully define the FROM statement of a query - this can be convenient if we look towards making our sources a bit more complex supporting things like table functions or whatnot.

I can imagine perhaps we'll adopt some template-like syntax so people can write a query like

SELECT count() FROM ${source} WHERE ...

and then select a visualization option for the returned data (line, table, etc.) as well as choosing which source they want the FROM statement to come from.

This will take a bit of work, our chart rendering reasonably does inference on the return table type and tries to interpret it automatically (picking out DateTime for x axis, categorical variables, etc.) but it might not be robust enough for arbitrary SQL queries.