Add virtual person properties defined by custom HogQL expressions

[robbie-c]

Feature request

Is your feature request related to a problem?

Some customers define the same expression in many queries, either as HogQL expressions in trends or as part of a HogQL query.

Describe the solution you'd like

It'd be cool if they could add them as a virtual property (similar to the experimental $virt_initial_channel_type on the persons model).

We'd need to make sure the property works in a few different scopes, e.g.

select persons.$my_virtual_property from persons;
select events.person.$my_virtual_property from events;

Describe alternatives you've considered

A lot of copy/pasting ;)

Additional context

It came up in this Zendesk ticket https://posthoghelp.zendesk.com/agent/tickets/22482

It'd be cool if this column could be used in batch exports too, it'd be a way for customers to add computed fields to that.