Add header to differentiate between assets on S3 vs NFS
Add header to differentiate between assets on S3 vs NFS

We're in the process of migrating assets from NFS to S3. The first step
is to proxy requests to assets hosted on S3. This header will help us by
providing information about where the asset is stored (NFS or S3).

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet