Allow accessing nested JSON properties in UI filters

Feature request

Is your feature request related to a problem?

The UI property filter picker doesn't natively support drilling into nested JSON objects. It only surfaces top-level properties.

Specifically, I'm trying to create a dynamic cohort based on a nested JSON attribute in a person property (e.g., profile.user_type) , but I can only select the top-level property (profile). The UI cohort config does not have a SQL expression option.

Describe the solution you'd like

1. Selecting a property that is a JSON object should then surface its attributes as filterable properties
   or
2. Make sure SQL is an option everywhere properties can be selected as filters/criteria

Describe alternatives you've considered

Flattening all JSON objects

Additional context

Debug info

Session: https://us.posthog.com/project/sTMFPsFhdP1Ssg/replay/019f75e6-9eb6-7dbb-9b27-19cf0a0f2b4f?t=2780
Admin (internal): http://go/adminOrgUS/01962554-e342-0000-4942-7fa64a8d395f (project ID 483452)

Start from the UI property filter picker and cohort configuration described in the issue, tracing how top-level JSON properties become selectable filters. Compare the two proposed approaches—surfacing nested attributes or enabling SQL criteria—and define completion as supporting nested JSON filtering in the relevant UI flow.