feat: add support for ph-label in autocapture elements

[ioannisj]

💡 Motivation and Context

Private slack thread: https://posthog.slack.com/archives/C03PB072FMJ/p1733219779409259
Docs: PostHog/posthog.com#10085

Autocapture elements can now be given a PostHog label which can then be used in analytics to filter for a specific element (using Tag Name and/or CSS Selector)

Autocapture elements now get class and id attributes:

• class: the superclass of the element if not a common type like UIView, UIControl, NSObject or UIResponder
• id: the value of the custom label defined through postHogLabel (UIKit) or .postHogLabel(_ label: String?) (SwiftUI)

SwiftUI

• I think most of the interactions in a SwiftUI app are already autocaptured (since most of interactive SwiftUI controls map to a UIKit counterpart). List includes collection views, scroll views, table views, text fields, text views, switches etc
• An important element we are missing is support for UIButton (SwiftUI renders buttons as images)

Minor

• Some minor improvements/corrections
• New simpler implementation of ForwardingPickerViewDelegate

Notes

The implementation uses $elements_chain (constructed by the client) since $elements will be deprecated soon https://github.com/PostHog/posthog/blob/f9be4fa28668e242d56bab9651cf9c79b40d9249/plugin-server/src/worker/ingestion/process-event.ts#L119

💚 How did you test it?

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.