Implement a browser-backend route for generating tarballs

Before raising this MR, consider whether the following are required, and complete if so:

• Unit tests
• Metrics
• Documentation update(s)

If not required, please explain in brief why not.

Description

This MR adds a route to bgd browser-backend which generates and responds with a gzipped tarball when given the digest of a Directory message. The tarball contains the contents of that directory fetched from the remote CAS.

Issues addressed

Closes #364 (closed)