This PR adds the ability to set HTTP headers when downloading
an artifact from an `http` or `https` resource.
The implementation in `go-getter` is such that a new `HTTPGetter`
must be created for each artifact that sets headers (as opposed
to conveniently setting headers per-request). This PR maintains
the memoization of the default Getter objects, creating new ones
only for artifacts where headers are set.
Closes#9306
Ensure that the client honors the client configuration for the
`template.disable_file_sandbox` field when validating the jobspec's
`template.source` parameter, and not just with consul-template's own `file`
function.
Prevent interpolated `template.source`, `template.destination`, and
`artifact.destination` fields from escaping file sandbox.
As part of deprecating legacy drivers, we're moving the env package to a
new drivers/shared tree, as it is used by the modern docker and rkt
driver packages, and is useful for 3rd party plugins.
