This PR adds support for configuring `proxy.upstreams[].config` for
Consul Connect upstreams. This is an opaque config value to Nomad -
the data is passed directly to Consul and is unknown to Nomad.
When configuring Consul Service Mesh, it's sometimes necessary to
provide dynamic value that are only known to Nomad at runtime. By
interpolating configuration values (in addition to configuration keys),
user are able to pass these dynamic values to Consul from their Nomad
jobs.