consul/connect: add support for running connect native tasks

This PR adds the capability of running Connect Native Tasks on Nomad,
particularly when TLS and ACLs are enabled on Consul.

The `connect` stanza now includes a `native` parameter, which can be
set to the name of task that backs the Connect Native Consul service.

There is a new Client configuration parameter for the `consul` stanza
called `share_ssl`. Like `allow_unauthenticated` the default value is
true, but recommended to be disabled in production environments. When
enabled, the Nomad Client's Consul TLS information is shared with
Connect Native tasks through the normal Consul environment variables.
This does NOT include auth or token information.

If Consul ACLs are enabled, Service Identity Tokens are automatically
and injected into the Connect Native task through the CONSUL_HTTP_TOKEN
environment variable.

Any of the automatically set environment variables can be overridden by
the Connect Native task using the `env` stanza.

Fixes #6083

vendor/github.com/hashicorp/nomad/api/services.go

@@ -140,7 +140,7 @@ func (s *Service) Canonicalize(t *Task, tg *TaskGroup, job *Job) {
 
 // ConsulConnect represents a Consul Connect jobspec stanza.
 type ConsulConnect struct {
-	Native         bool
+	Native         string
 	SidecarService *ConsulSidecarService `mapstructure:"sidecar_service"`
 	SidecarTask    *SidecarTask          `mapstructure:"sidecar_task"`
 }