Support SFTP paths to root or home directory

[jgogstad]

This PR introduces a new test container that configures the SFTP server without the ChrootDirectory directive. This causes users to land in /home/<username>/ instead of / (ChrootDirectory %h maps the user's home directory to /).

In order for tests to pass on both server configuration, we must support relative paths. This PR introduces that support.

Discussion, not strictly related to PR contents

I think the Path abstraction needs to be augmented or adjusted in order to fully support SFTP. It currently leaks assumptions about the underlying storage, i.e. there's always two components to paths (root and key) and there's no separation between absolute and relative paths. This doesn't hold up for SFTP stores where the following paths are all valid and pointing to different files (ref spec#section-6):

1. Absolute path "/foo.txt"
2. Relative to CWD path "./foo.txt"
3. Relative to user home path "foo.txt"

Since we don't surface any APIs for changing CWD, we can probably get away with assuming CWD == user home.

Any views on a better API is welcome. I think we should encode the fact that blobstore paths are different from filestore paths (e.g. SFTP) and introduce something like

sealed trait Path
object Path {
  case class BlobstorePath(scheme:String, bucketName: String, path: String) extends Path
  case class FilestorePath(path: String) extends Path
}

trait Store[F[_]] {
  type PathType
}

[rolandomanrique]

The path abstraction is certainly hard to generalize and still support all stores native features (e.g. s3 store must have size on put or could support per path encryption).

We did make that compromise that if you need different settings per file Path you would need different Store instances (at LendUp I had to do this for files with different encryption algorithms).

Our use case for SFTP (and also box.com store) was that we had to restrict users to a specific dir just like a “bucket” so this is why we never added support for abs paths.

All these assumptions can certainly be revised and improved.

[codecov]

Codecov Report

Merging #5 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master       #5   +/-   ##
=======================================
  Coverage   67.57%   67.57%           
=======================================
  Files           8        8           
  Lines         293      293           
  Branches       11        8    -3     
=======================================
  Hits          198      198           
  Misses         95       95           

Flag	Coverage Δ
#scala_2_12_10	67.57% <100.00%> (ø)
#scala_2_13_1	67.58% <100.00%> (+0.22%)	⬆️

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[jgogstad]

Just formatting here