[misc] Move shared components to shared directory (#4286)

Moved the following directories:

```
  - management/client → shared/management/client
  - management/domain → shared/management/domain
  - management/proto → shared/management/proto
  - signal/client → shared/signal/client
  - signal/proto → shared/signal/proto
  - relay/client → shared/relay/client
  - relay/auth → shared/relay/auth
```

and adjusted import paths

shared/management/domain/list.go

@@ -0,0 +1,108 @@
+package domain
+
+import (
+	"sort"
+	"strings"
+)
+
+// List is a slice of punycode-encoded domain strings.
+type List []Domain
+
+// ToStringList converts a List to a slice of string.
+func (d List) ToStringList() ([]string, error) {
+	var list []string
+	for _, domain := range d {
+		s, err := domain.String()
+		if err != nil {
+			return nil, err
+		}
+		list = append(list, s)
+	}
+	return list, nil
+}
+
+// ToPunycodeList converts the List to a slice of Punycode-encoded domain strings.
+func (d List) ToPunycodeList() []string {
+	var list []string
+	for _, domain := range d {
+		list = append(list, string(domain))
+	}
+	return list
+}
+
+// ToSafeStringList converts the List to a slice of non-punycode strings.
+// If a domain cannot be converted, the original string is used.
+func (d List) ToSafeStringList() []string {
+	var list []string
+	for _, domain := range d {
+		list = append(list, domain.SafeString())
+	}
+	return list
+}
+
+// String converts List to a comma-separated string.
+func (d List) String() (string, error) {
+	list, err := d.ToStringList()
+	if err != nil {
+		return "", err
+	}
+	return strings.Join(list, ", "), nil
+}
+
+// SafeString converts List to a comma-separated non-punycode string.
+// If a domain cannot be converted, the original string is used.
+func (d List) SafeString() string {
+	str, err := d.String()
+	if err != nil {
+		return d.PunycodeString()
+	}
+	return str
+}
+
+// PunycodeString converts the List to a comma-separated string of Punycode-encoded domains.
+func (d List) PunycodeString() string {
+	return strings.Join(d.ToPunycodeList(), ", ")
+}
+
+func (d List) Equal(domains List) bool {
+	if len(d) != len(domains) {
+		return false
+	}
+
+	sort.Slice(d, func(i, j int) bool {
+		return d[i] < d[j]
+	})
+
+	sort.Slice(domains, func(i, j int) bool {
+		return domains[i] < domains[j]
+	})
+
+	for i, domain := range d {
+		if domain != domains[i] {
+			return false
+		}
+	}
+	return true
+}
+
+// FromStringList creates a DomainList from a slice of string.
+func FromStringList(s []string) (List, error) {
+	var dl List
+	for _, domain := range s {
+		d, err := FromString(domain)
+		if err != nil {
+			return nil, err
+		}
+		dl = append(dl, d)
+	}
+	return dl, nil
+}
+
+// FromPunycodeList creates a List from a slice of Punycode-encoded domain strings.
+func FromPunycodeList(s []string) List {
+	var dl List
+	for _, domain := range s {
+		dl = append(dl, Domain(strings.ToLower(domain)))
+	}
+	return dl
+}