feat: add terramate.stack.parent.id. (#1955)

## What this PR does / why we need it:

Implements `terramate.stack.parent.id` metadata for stacks that has
parent stacks.

## Which issue(s) this PR fixes:
none

## Special notes for your reviewer:

## Does this PR introduce a user-facing change?
```
yes, add a new feature
```

Author: i4ki

CHANGELOG.md

@@ -22,6 +22,10 @@ Given a version number `MAJOR.MINOR.PATCH`, we increment the:
 
 ## Unreleased
 
+### Added
+
+- Add `terramate.stack.parent.id` metadata to stacks that are part of a parent-child hierarchy.
+
 ## v0.11.1
 
 ### Added

config/stack.go

@@ -236,6 +236,21 @@ func (s *Stack) RuntimeValues(root *Root) map[string]cty.Value {
 	if s.ID != "" {
 		stackMapVals["id"] = cty.StringVal(s.ID)
 	}
+	cfg, _ := root.Lookup(s.Dir)
+	var parentStack *Tree
+
+	for cfg.Parent != nil {
+		cfg = cfg.Parent
+		if cfg.IsStack() {
+			parentStack = cfg
+			break
+		}
+	}
+	if parentStack != nil {
+		stackMapVals["parent"] = cty.ObjectVal(map[string]cty.Value{
+			"id": cty.StringVal(parentStack.Node.Stack.ID),
+		})
+	}
 	stack := cty.ObjectVal(stackMapVals)
 	return map[string]cty.Value{
 		"name":        cty.StringVal(s.Name),         // DEPRECATED

e2etests/core/debug_show_metadata_test.go

@@ -246,6 +246,72 @@ stack "/stack":
 	terramate.stack.path.basename="stack"
 	terramate.stack.path.relative="stack"
 	terramate.stack.path.to_root=".."
+`,
+			},
+		},
+		{
+			name: "one stack with parent stack",
+			layout: []string{
+				`s:parent:id=parent-stack`,
+				`s:parent/child`,
+			},
+			want: RunExpected{
+				Stdout: `Available metadata:
+
+project metadata:
+	terramate.stacks.list=[/parent /parent/child]
+
+stack "/parent":
+	terramate.stack.id="parent-stack"
+	terramate.stack.name="parent"
+	terramate.stack.description=""
+	terramate.stack.tags=[]
+	terramate.stack.path.absolute="/parent"
+	terramate.stack.path.basename="parent"
+	terramate.stack.path.relative="parent"
+	terramate.stack.path.to_root=".."
+
+stack "/parent/child":
+	terramate.stack.name="child"
+	terramate.stack.description=""
+	terramate.stack.tags=[]
+	terramate.stack.path.absolute="/parent/child"
+	terramate.stack.path.basename="child"
+	terramate.stack.path.relative="parent/child"
+	terramate.stack.path.to_root="../.."
+`,
+			},
+		},
+		{
+			name: "one stack with parent stack separated by directories",
+			layout: []string{
+				`s:parent:id=parent-stack`,
+				`s:parent/some/child/stack`,
+			},
+			want: RunExpected{
+				Stdout: `Available metadata:
+
+project metadata:
+	terramate.stacks.list=[/parent /parent/some/child/stack]
+
+stack "/parent":
+	terramate.stack.id="parent-stack"
+	terramate.stack.name="parent"
+	terramate.stack.description=""
+	terramate.stack.tags=[]
+	terramate.stack.path.absolute="/parent"
+	terramate.stack.path.basename="parent"
+	terramate.stack.path.relative="parent"
+	terramate.stack.path.to_root=".."
+
+stack "/parent/some/child/stack":
+	terramate.stack.name="stack"
+	terramate.stack.description=""
+	terramate.stack.tags=[]
+	terramate.stack.path.absolute="/parent/some/child/stack"
+	terramate.stack.path.basename="stack"
+	terramate.stack.path.relative="parent/some/child/stack"
+	terramate.stack.path.to_root="../../../.."
 `,
 			},
 		},

e2etests/core/run_sharing_test.go

@@ -71,6 +71,52 @@ func TestRunSharing(t *testing.T) {
 				assert.EqualStrings(t, expected, string(s.RootEntry().ReadFile("s2/file.txt")))
 			},
 		},
+		{
+			name: "parent/child sharing",
+			layout: []string{
+				"f:backend.tm:" + Block("sharing_backend",
+					Labels("name"),
+					Expr("type", "terraform"),
+					Str("filename", "sharing.tf"),
+					Command("terraform", "output", "-json"),
+				).String(),
+				"s:s1:id=s1",
+				"f:s1/main.tf:" + Doc(
+					Block("resource",
+						Labels("local_file", "s1_file"),
+						Str("content", "s1_content"),
+						Str("filename", "${path.module}/file.txt"),
+					),
+				).String(),
+				"f:s1/output.tm:" + Output(
+					Labels("s1_output"),
+					Str("backend", "name"),
+					Expr("value", "resource.local_file.s1_file.content"),
+				).String(),
+				"s:s1/s2",
+				"f:s1/s2/input.tm:" + Input(
+					Labels("s2_input"),
+					Str("backend", "name"),
+					Expr("value", "outputs.s1_output.value"),
+					Expr("from_stack_id", `terramate.stack.parent.id`),
+				).String(),
+				"f:s1/s2/main.tf:" + Doc(
+					Block("resource",
+						Labels("local_file", "s2_file"),
+						Expr("content", "var.s2_input"),
+						Str("filename", "${path.module}/file.txt"),
+					),
+				).String(),
+			},
+			check: func(t *testing.T, s *sandbox.S, res RunResult) {
+				AssertRunResult(t, res, RunExpected{
+					IgnoreStdout: true,
+				})
+				const expected = "s1_content"
+				assert.EqualStrings(t, expected, string(s.RootEntry().ReadFile("s1/file.txt")))
+				assert.EqualStrings(t, expected, string(s.RootEntry().ReadFile("s1/s2/file.txt")))
+			},
+		},
 		{
 			name: "input with no output counterpart",
 			layout: []string{