Allow selecting from collections with inconsistent properties

[ropjov]

Is your feature request related to a problem? Please describe.
I am trying to visualise some of my Firestore data with Grafana. But the issue is that the collection I am trying to query using FireQL does not have a consistent set of properties, since it's collecting daily data, and not every day may have all data.

For example:
SELECT * FROM inconsistent_property_collection throws fireql.Execute: unknown field "inconsistentProperty" in doc "2023-10-18"

Describe the solution you'd like
I think allowing properties to not be consistent, and considering them a SQL NULL when they are not present would make this library, and specially the usage from Grafana that much more useful. Otherwise, most Firestore-like collections won't work with FireQL, since Firestore does not enforce consistent properties.

Also, to keep back-wards compatibility this new behaviour could require an opt-in, and could use an SQL-like syntax. For example:
SET allow_inconsistent_properties = true; SELECT * FROM inconsistent_property_collection

or

SELECT * FROM inconsistent_property_collection ALLOW undefined_properties

Describe alternatives you've considered
The main alternative I have considered is to stop using FireQL and Firestore and use Postgres. I don't see a way to get this working with my type of collections.

Additional context
All of this was tested using the latest version of the Grafana Firestore integration