feat(hogql): Expression Fields & PoEv2

[mariusandra]

Dependent on #18725

Problem

We need support for custom SQL expressions in HogQL tables.

1. Accessing person_id on events with PoEv2 has to evaluate to something like override.override_person_id or events.person_id
2. We'll want custom source and other columns for web analytics.
3. We eventually want to offer calculated properties as a first class feature to users.

Changes

• Adds a new database field type, "Expression Fields", which pretend to be regular database fields, but swap out their contents when resolving under ClickHouse.
• Updates "person_id" and person.id when PoEv2 is enabled, makes it use the override vs old_id code.
• Wiring to make this all work.

Known limitation

The current expression fields have no concept of this. With events.a = event, when running select distinct_id as event, a from events, we'd return the distinct_id twice, instead of a resolving to the event field, which was the only one in scope when the expression field was defined.

This will be fixed in a followup, but is non-blocking today. The biggest problem this can cause is a "error redefining alias" message or an otherwise erroring query, such as not knowing which person_id to choose if there are many in scope.

Effectively this means we can play around with these custom fields, but we shouldn't release a "calculated properties" feature to users today.

How did you test this code?

Wrote tests. Tested the new lifecycle query with PoEv2 enabled, everything just worked on the first go. It was magical.

[Gilbert09]

🚀