Problem: processing workflow has separate code paths for filesystem and minio watchers

[djjuhasz]

Describe the problem

The processing workflow and bundle activity use conditional branches to manage differences between the filesystem watcher and minio watcher implementations. The branching logic is hard to understand, requires some code duplication and makes the assumption that only a filesystem watcher can provide a "blob" that is a directory.

Possible solutions

The differences between the two watchers could be managed by moving the difference in implementation for copying the blob data to a local "processing" directory into their individual implementations instead of requiring branching logic (see "Avoiding conditionals by obeying the Liskov Substitution Principle" - https://sandimetz.com/99bottles).

Additional context

• https://github.com/artefactual-sdps/enduro/blob/main/internal/workflow/processing.go#L295 branches based on whether tinfo.req.IsDir is true (which is only possible for a filesystem watcher) or false (possible for both minio and filesystem watchers)
• https://github.com/artefactual-sdps/enduro/blob/main/internal/workflow/activities/bundle.go#L89 branches based on whether param.IsDir (a local copy of tinfo.req.IsDir) is true. This branch requires duplicate code for creating a temporary dir in the transfer dir, and stripping a top-level directory
• the current implementation assumes that a SIP uploaded to MinIO can not be a directory of files, but we may want to change this in the future (see #845)

[djjuhasz]

This is complicated by the fact that the current watcher.Service.Download() method takes an io.Writer parameter as the destination to write the byte stream. An io.Writer stream works for a single file, as we expect from a minioWatcher, but doesn't work for a filesystem directory as we might receive from a filesystemWatcher.