jobspec: allow artifact headers in HCLv1 (#14637)

* jobspec: allow artifact headers in HCLv1

Co-authored-by: Luiz Aoqui <luiz@hashicorp.com>

.changelog/14637.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+jobspec: Fixed a bug where an `artifact` with `headers` configuration would fail to parse when using HCLv1
+```

jobspec/parse_task.go

@@ -370,6 +370,7 @@ func parseArtifacts(result *[]*api.TaskArtifact, list *ast.ObjectList) error {
 		valid := []string{
 			"source",
 			"options",
+			"headers",
 			"mode",
 			"destination",
 		}

jobspec/parse_test.go

@@ -628,6 +628,13 @@ func TestParse(t *testing.T) {
 										GetterOptions: nil,
 										RelativeDest:  stringToPtr("var/foo"),
 									},
+									{
+										GetterSource: stringToPtr("https://example.com/file.txt"),
+										GetterHeaders: map[string]string{
+											"User-Agent":    "nomad",
+											"X-Nomad-Alloc": "alloc",
+										},
+									},
 								},
 							},
 						},

jobspec/test-fixtures/artifacts.hcl

@@ -20,6 +20,15 @@ job "binstore-storagelocker" {
         source      = "http://foo.com/bam"
         destination = "var/foo"
       }
+
+      artifact {
+        source = "https://example.com/file.txt"
+
+        headers {
+          User-Agent    = "nomad"
+          X-Nomad-Alloc = "alloc"
+        }
+      }
     }
   }
 }