Allow specifying extended MAPI properties when creating a message (#15)

There's more code duplication than I'm comfortable with in there, but I don't really see a way out of it - see thunderbird/xml-struct-rs#9 which describes the issue in more details.

I have also made `message_disposition` not an `Option`, since as far as I'm aware this is a required field.

src/types/common.rs

@@ -124,6 +124,41 @@ pub enum PathToElement {
     },
 }
 
+/// The identifier for an extended MAPI property.
+///
+/// See <https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/extendedfielduri>
+// N.B.: This is copied from `PathToElement::ExtendedFieldURI`,
+// which follows the same structure. However, xml-struct doesn't currently
+// support using a nested structure to define an element's attributes, see
+// https://github.com/thunderbird/xml-struct-rs/issues/9
+#[derive(Debug, XmlSerialize)]
+pub struct ExtendedFieldURI {
+    /// A well-known identifier for a property set.
+    #[xml_struct(attribute)]
+    pub distinguished_property_set_id: Option<DistinguishedPropertySet>,
+
+    /// A GUID representing a property set.
+    // TODO: This could use a strong type for representing a GUID.
+    #[xml_struct(attribute)]
+    pub property_set_id: Option<String>,
+
+    /// Specifies a property by integer tag.
+    #[xml_struct(attribute)]
+    pub property_tag: Option<String>,
+
+    /// The name of a property within a specified property set.
+    #[xml_struct(attribute)]
+    pub property_name: Option<String>,
+
+    /// The dispatch ID of a property within a specified property set.
+    #[xml_struct(attribute)]
+    pub property_id: Option<String>,
+
+    /// The value type of the desired property.
+    #[xml_struct(attribute)]
+    pub property_type: PropertyType,
+}
+
 /// A well-known MAPI property set identifier.
 ///
 /// See <https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/extendedfielduri#distinguishedpropertysetid-attribute>

src/types/create_item.rs

@@ -6,8 +6,8 @@ use serde::Deserialize;
 use xml_struct::XmlSerialize;
 
 use crate::{
-    types::sealed::EnvelopeBodyContents, ArrayOfRecipients, BaseFolderId, Items, MimeContent,
-    Operation, OperationResponse, ResponseClass, ResponseCode, MESSAGES_NS_URI,
+    types::sealed::EnvelopeBodyContents, ArrayOfRecipients, BaseFolderId, ExtendedFieldURI, Items,
+    MimeContent, Operation, OperationResponse, ResponseClass, ResponseCode, MESSAGES_NS_URI,
 };
 
 /// The action an Exchange server will take upon creating a `Message` item.
@@ -35,7 +35,7 @@ pub struct CreateItem {
     ///
     /// See <https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/createitem#messagedisposition-attribute>
     #[xml_struct(attribute)]
-    pub message_disposition: Option<MessageDisposition>,
+    pub message_disposition: MessageDisposition,
 
     /// The folder in which to store an item once it has been created.
     ///
@@ -84,16 +84,36 @@ pub struct Message {
     /// The MIME content of the item.
     #[xml_struct(ns_prefix = "t")]
     pub mime_content: Option<MimeContent>,
+
     // Whether to request a delivery receipt.
     #[xml_struct(ns_prefix = "t")]
     pub is_delivery_receipt_requested: Option<bool>,
+
     // The message ID for the message, semantically identical to the Message-ID
     // header.
     #[xml_struct(ns_prefix = "t")]
     pub internet_message_id: Option<String>,
+
     // Recipients to include as Bcc, who won't be included in the MIME content.
     #[xml_struct(ns_prefix = "t")]
     pub bcc_recipients: Option<ArrayOfRecipients>,
+
+    // Extended MAPI properties to set on the message.
+    #[xml_struct(ns_prefix = "t")]
+    pub extended_property: Option<Vec<ExtendedProperty>>,
+}
+
+/// An extended MAPI property to set on the message.
+///
+/// See <https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/extendedproperty>
+#[allow(non_snake_case)]
+#[derive(Debug, XmlSerialize)]
+pub struct ExtendedProperty {
+    #[xml_struct(ns_prefix = "t")]
+    pub extended_field_URI: ExtendedFieldURI,
+
+    #[xml_struct(ns_prefix = "t")]
+    pub value: String,
 }
 
 impl Operation for CreateItem {