Support for reporting progress of long-running queries

[maxalbert]

It would be useful to have better support for reporting progress / estimated duration until completion for long-running queries. Use cases are UIs that interact with FlowKit via the API (e.g. for visualisation) as well as debugging, code development and optimisation (to get a better feel for how queries scale).

As far as I'm aware PostgreSQL doesn't provide a lot of built-in support for this, but it is possible that we can provide some custom scaffolding for certain query types - e.g. if a large query is composed of smaller ones and these are cached then we can determine how many of them have already been calculated and estimate the progress this way.

(On a tangential note, a similar approach may also be useful when starting up flowdb containers with large sample data - we could add support for querying how many days of sample data have already been generated and display a progress bar based on this.)

I'm not sure how much we'll be able to do in this regard, but especially for user feedback through the API it would be good to have some support.