R1:c0b8e4784bfa

Add a basic, general-purpose export workflow for all objects with SearchEngine support

Summary:
Depends on D18918. Ref T13046. Ref T5954. Pull logs can currently be browsed in the web UI, but this isn't very powerful, especially if you have thousands of them.

Allow SearchEngine implementations to define exportable fields so that users can "Use Results > Export Data" on any query. In particular, they can use this workflow to download a file with pull logs.

In the future, this can replace the existing…