[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/domain.go

@@ -0,0 +1,45 @@
+package domain
+
+import (
+	"strings"
+
+	"golang.org/x/net/idna"
+)
+
+// Domain represents a punycode-encoded domain string.
+// This should only be converted from a string when the string already is in punycode, otherwise use FromString.
+type Domain string
+
+// String converts the Domain to a non-punycode string.
+// For an infallible conversion, use SafeString.
+func (d Domain) String() (string, error) {
+	unicode, err := idna.ToUnicode(string(d))
+	if err != nil {
+		return "", err
+	}
+	return unicode, nil
+}
+
+// SafeString converts the Domain to a non-punycode string, falling back to the punycode string if conversion fails.
+func (d Domain) SafeString() string {
+	str, err := d.String()
+	if err != nil {
+		return string(d)
+	}
+	return str
+}
+
+// PunycodeString returns the punycode representation of the Domain.
+// This should only be used if a punycode domain is expected but only a string is supported.
+func (d Domain) PunycodeString() string {
+	return string(d)
+}
+
+// FromString creates a Domain from a string, converting it to punycode.
+func FromString(s string) (Domain, error) {
+	ascii, err := idna.ToASCII(s)
+	if err != nil {
+		return "", err
+	}
+	return Domain(strings.ToLower(ascii)), nil
+}