[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/validate.go

@@ -0,0 +1,56 @@
+package domain
+
+import (
+	"fmt"
+	"regexp"
+	"strings"
+)
+
+const maxDomains = 32
+
+var domainRegex = regexp.MustCompile(`^(?:\*\.)?(?:(?:xn--)?[a-zA-Z0-9_](?:[a-zA-Z0-9-_]{0,61}[a-zA-Z0-9])?\.)*(?:xn--)?[a-zA-Z0-9](?:[a-zA-Z0-9-_]{0,61}[a-zA-Z0-9])?$`)
+
+// ValidateDomains checks if each domain in the list is valid and returns a punycode-encoded DomainList.
+func ValidateDomains(domains []string) (List, error) {
+	if len(domains) == 0 {
+		return nil, fmt.Errorf("domains list is empty")
+	}
+	if len(domains) > maxDomains {
+		return nil, fmt.Errorf("domains list exceeds maximum allowed domains: %d", maxDomains)
+	}
+
+	var domainList List
+
+	for _, d := range domains {
+		// handles length and idna conversion
+		punycode, err := FromString(d)
+		if err != nil {
+			return domainList, fmt.Errorf("convert domain to punycode: %s: %w", d, err)
+		}
+
+		if !domainRegex.MatchString(string(punycode)) {
+			return domainList, fmt.Errorf("invalid domain format: %s", d)
+		}
+
+		domainList = append(domainList, punycode)
+	}
+	return domainList, nil
+}
+
+// ValidateDomainsList checks if each domain in the list is valid
+func ValidateDomainsList(domains []string) error {
+	if len(domains) == 0 {
+		return nil
+	}
+	if len(domains) > maxDomains {
+		return fmt.Errorf("domains list exceeds maximum allowed domains: %d", maxDomains)
+	}
+
+	for _, d := range domains {
+		d := strings.ToLower(d)
+		if !domainRegex.MatchString(d) {
+			return fmt.Errorf("invalid domain format: %s", d)
+		}
+	}
+	return nil
+}