jobspec: support cluster field for Vault block (#18408)

This field supports the upcoming ENT-only multiple Vault clusters feature. The
job validation and mutation hooks will come in a separate PR.

Ref: https://github.com/hashicorp/team-nomad/issues/404

command/agent/job_endpoint.go

@@ -1292,6 +1292,7 @@ func ApiTaskToStructsTask(job *structs.Job, group *structs.TaskGroup,
 			Role:         apiTask.Vault.Role,
 			Policies:     apiTask.Vault.Policies,
 			Namespace:    *apiTask.Vault.Namespace,
+			Cluster:      apiTask.Vault.Cluster,
 			Env:          *apiTask.Vault.Env,
 			DisableFile:  *apiTask.Vault.DisableFile,
 			ChangeMode:   *apiTask.Vault.ChangeMode,

command/agent/job_endpoint_test.go

@@ -3214,6 +3214,7 @@ func TestJobs_ApiJobToStructsJob(t *testing.T) {
 						Vault: &structs.Vault{
 							Role:         "nomad-task",
 							Namespace:    "ns1",
+							Cluster:      "default",
 							Policies:     []string{"a", "b", "c"},
 							Env:          true,
 							DisableFile:  false,