Epic: Consume posthog API from storage controller and pageserver

[jcsp]

Posthog is a system for managing feature flags, amongst other things.

Currently when we do a "feature flag" for a storage feature, it's really a configuration attribute on either PageserverConf or TenantConf. These are cumbersome to manage, as they're really intended more for long term configurable things than for lots of turnover in settings coming and going. They're also cumbersome to change, as they can only be toggled during deploys.

An API like Posthog's is appropriate for relatively rare operations, where the cost of checking flags is not too high -- for example compaction, serving a basebackup. It is not appropriate for checking on very frequent operations, such as getpage requests. Any featured flagged behavior during getpage requests would need to be loaded earlier, for example during tenant attach.