Add artifactory storage backend

[bored-engineer]

What is the problem I am trying to address?

Add the option to use Artifactory as a storage backend for athens.
The implementation it is very similar to the existing external provider with a few tweaks specific to the Artifactory REST API and specific configuration options added for usability.
Note: This client uses artifactory as a generic object storage backend, it may or may not be compatible with the folder structure created by official Artifactory Go Registry plugin.

How is the fix applied?

Implements a new storage.Backend which uses the Artifactory REST API to read/write objects.
The upload action is accomplished by writing a zip file (generated on the fly) and using the X-Explode-Archive: true and X-Explode-Archive-Atomic: true so the objects are extracted atomically into a folder by artifactory.
Lister functionality is accomplished by the FolderInfo API call.

What GitHub issue(s) does this PR fix or close?

Fixes #1130 (which was already closed/abandoned but is the closest match)

[bored-engineer]

redis: 2020/10/18 19:45:13 sentinel.go:393: sentinel: GetMasterAddrByName master="redis-1" failed: dial tcp: lookup redis-sentinel on 127.0.0.11:53: no such host
--
429 | with_redis_sentinel_test.go:31: redis: all sentinels are unreachable

Seems like the test failure is unrelated to the PR

[arschles]

@bored-engineer yea, I think that's a flake. I restarted it. One question though - would it not be better to just use Artifactory as an upstream? iirc you can use it as a Go module server as well

[bored-engineer]

One question though - would it not be better to just use Artifactory as an upstream? iirc you can use it as a Go module server as well

Yes, that would absolutely make sense if that's an available in your environment. JFrog offers a Go Registry that should be the first option to use.

In our environment that was not an option but using Artifactory as a storage backend was a business requirement (due to in-built compliance/security controls).

[arschles]

@bored-engineer makes sense. Ok I will review this right now.

[arschles]

@bored-engineer this looks very well done, thanks for submitting it. The only thing left would be to write some tests.