Add round-trip conversion initializers for RFC email address types

- Add RFC_5321.EmailAddress.init(_ emailAddress: EmailAddress)
- Add RFC_5322.EmailAddress.init(_ emailAddress: EmailAddress)
- Add RFC_6531.EmailAddress.init(_ emailAddress: EmailAddress)

These initializers enable cleaner conversion syntax:
  let rfc5322 = try RFC_5322.EmailAddress(emailAddress)
Instead of:
  let rfc5322 = try RFC_5322.EmailAddress(emailAddress.rawValue)

Placement in swift-emailaddress-type avoids creating reverse dependencies
in RFC packages while providing convenient conversion APIs.

Author: coenttb

Sources/EmailAddress/EmailAddress+RFC5321.swift

@@ -27,3 +27,18 @@ extension EmailAddress {
         self.displayName = rfc5321.displayName
     }
 }
+
+extension RFC_5321.EmailAddress {
+    /// Initialize from EmailAddress
+    ///
+    /// Enables round-trip conversion between EmailAddress and RFC_5321.EmailAddress.
+    ///
+    /// - Parameter emailAddress: The EmailAddress to convert
+    /// - Throws: If the EmailAddress doesn't have a valid RFC 5321 representation
+    public init(_ emailAddress: EmailAddress) throws {
+        guard let rfc5321 = emailAddress.rfc5321 else {
+            throw EmailAddress.Error.conversionFailure
+        }
+        self = rfc5321
+    }
+}

Sources/EmailAddress/EmailAddress+RFC5322.swift

@@ -23,3 +23,19 @@ extension EmailAddress {
         self.displayName = rfc5322.displayName
     }
 }
+
+extension RFC_5322.EmailAddress {
+    /// Initialize from EmailAddress
+    ///
+    /// Enables round-trip conversion between EmailAddress and RFC_5322.EmailAddress.
+    /// This is particularly useful when converting Email objects to RFC 5322 Message format.
+    ///
+    /// - Parameter emailAddress: The EmailAddress to convert
+    /// - Throws: If the EmailAddress doesn't have a valid RFC 5322 representation
+    public init(_ emailAddress: EmailAddress) throws {
+        guard let rfc5322 = emailAddress.rfc5322 else {
+            throw EmailAddress.Error.conversionFailure
+        }
+        self = rfc5322
+    }
+}

Sources/EmailAddress/EmailAddress+RFC6531.swift

@@ -12,3 +12,15 @@ extension EmailAddress {
         self.displayName = rfc6531.displayName
     }
 }
+
+extension RFC_6531.EmailAddress {
+    /// Initialize from EmailAddress
+    ///
+    /// Enables round-trip conversion between EmailAddress and RFC_6531.EmailAddress.
+    /// RFC 6531 is the most permissive format and supports internationalized email addresses.
+    ///
+    /// - Parameter emailAddress: The EmailAddress to convert
+    public init(_ emailAddress: EmailAddress) {
+        self = emailAddress.rfc6531
+    }
+}