Support Workhorse S3 client credentials configuration

In gitlab-workhorse!466 (diffs), we will be adding configuration to Workhorse to access S3 directly instead of depending on Rails. The new config.toml will look something like:

[redis]
URL = "unix:/home/git/gitlab/redis/redis.socket"

[object_storage]
  enabled = true
  provider = "AWS"

[object_storage.s3]
  aws_access_key_id = "YOUR AWS ACCESS KEY"
  aws_secret_access_key = "YOUR AWS SECRET ACCESS KEY"