As we are going to add a search tab in T185927, we want to make some changes to eventlogging SearchFunnel on iOS app to figure out the usage of it. Specifically, we'd like to:

• Rename the schema to MobileWikiAppiOSSearch. After discussing with @mpopov, we agree that it would be less confusing and easier to maintain when iOS and Android using different schemas.
• Add a source field to identify the source from which the search interface was invoked: the top of feed, the search tab, or from the article view
• Add a langswitch action when users tap on a different language on the search interface
• Add a position field to log the position of the search result that was clicked, from the list that was presented to the user
• Add is_anon, primary_language, session_id fields to standardize it with other iOS eventlogging schemas
• Rename other fields from camelCase to snake_case because our database are case insensitive
• We'd like to send an results action not only when new search results are retrieved by API, but also when cached results are shown. In v6.0.0, we don't cache results of last search any more.

Schema: https://meta.wikimedia.org/wiki/Schema:MobileWikiAppiOSSearch
Revision number: 18132115

We'd also like to add a search_tab field to MobileWikiAppiOSUserHistory to log whether the app opens to the new search tab.

This task depends on T185927. If there is any design change in T185927, the spec may need to be changed accordingly.