Create a helper to go from a reason to a reason with the metadata service of the original reason (#5092)

Will be used in later PRs to help make long-running streamstore
snapshots safer to create if they ever go past the kopia checkpoint
interval

---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

- [x] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Issue(s)

merge after:
* #5091

#### Test Plan

- [ ] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

src/pkg/backup/identity/identity.go

@@ -17,6 +17,8 @@ type Reasoner interface {
 	// SubtreePath returns the path prefix for data in existing backups that have
 	// parameters (tenant, protected resourced, etc) that match this Reasoner.
 	SubtreePath() (path.Path, error)
+	// ToMetadata returns the corresponding metadata reason for this reason.
+	ToMetadata() Reasoner
 }
 
 func NewReason(
@@ -67,3 +69,11 @@ func (r reason) SubtreePath() (path.Path, error) {
 
 	return p, clues.Wrap(err, "building path").OrNil()
 }
+
+func (r reason) ToMetadata() Reasoner {
+	return NewReason(
+		r.Tenant(),
+		r.ProtectedResource(),
+		r.Service().ToMetadata(),
+		r.Category())
+}