R1:e2296a0ff773

Modernize file storage engine selection

Summary:
Fixes T5843. File storage engines use a very old "selector" mechanism which makes them difficult to extend.

This mechanism predates widespread use of `PhutilSymbolLoader` to discover available implementations at runtime. Runtime discovery has generally proven more flexible and easier to use than explicit selection (although it sometimes needs more UI to support it in cases where order or enabled/disabled flags can not be directly determined).

Use a modern…