When working with `stdin` and `stdout`, the `addExtension` function
should return std::nullopt, since concatenating files is undesirable.
This fixes a bug where the output of the `StringMap` container would
have the index file appended to it, rendering an invalid tar.
Due to the removal of `S3StorageClient`, simplify the infrastructure
under `/Storage/`:
* Move and rename `LocalStorageClient` to `StorageClient`, de-virtualize
all of its methods
* Move and rename `LocalWritableFile`, `LocalReadableFile` to
`WritableFile` and `ReadableFile`, de-vitualize all methods
* Drop the `Std{in,out}StorageClient`, make `FilePath` handle the
stdin/stdout logic
* Remove `setStorageCredentials` from `PipelineManager`
Giacomo Vercesi