Use flattened structs for deserializing common values

[leftmostcat]

Instead of repeating properties which form a common base, we should use structs to encapsulate these and serde(flatten) to flatten them into the containing structure.

An initial attempt at this (definitions below) with the following error:

Deserialize(Custom("invalid type: map, expected a string"))`

/// An attachment to an Exchange item.
///
/// See <https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/attachments-ex15websvcsotherref>
#[derive(Debug, Deserialize)]
pub enum Attachment {
    /// An attachment containing an Exchange item.
    ///
    /// See <https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/itemattachment>
    #[serde(rename_all = "PascalCase")]
    ItemAttachment {
        #[serde(flatten)]
        info: AttachmentCommon,
        // XXX: With this field in place, parsing will fail if there is no
        // `AttachmentItem` in the response.
        // See https://github.com/tafia/quick-xml/issues/683
        // /// The attached item.
        // #[serde(rename = "$value")]
        // content: Option<AttachmentItem>,
    },

    /// An attachment containing a file.
    ///
    /// See <https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/fileattachment>
    #[serde(rename_all = "PascalCase")]
    FileAttachment {
        #[serde(flatten)]
        info: AttachmentCommon,

        /// Whether the attachment represents a contact photo.
        ///
        /// See <https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/iscontactphoto>
        is_contact_photo: Option<bool>,

        /// The base64-encoded content of the attachment.
        ///
        /// See <https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/content>
        content: Option<String>,
    },
}

#[derive(Debug, Deserialize)]
#[serde(rename_all = "PascalCase")]
pub struct AttachmentCommon {
    /// An identifier for the attachment.
    pub attachment_id: AttachmentId,

    /// The name of the attachment.
    ///
    /// See <https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/name-attachmenttype>
    pub name: String,

    /// The MIME type of the attachment's content.
    ///
    /// See <https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/contenttype>
    pub content_type: String,

    /// An arbitrary identifier for the attachment.
    ///
    /// This field is not set by Exchange and is intended for use by
    /// external applications.
    ///
    /// See <https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/contentid>
    pub content_id: Option<String>,

    /// A URI representing the location of the attachment's content.
    ///
    /// See <https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/contentlocation>
    pub content_location: Option<String>,

    /// The size of the attachment's content in bytes.
    ///
    /// See <https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/size>
    pub size: Option<usize>,

    /// The most recent modification time for the attachment.
    ///
    /// See <https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/lastmodifiedtime>
    pub last_modified_time: Option<DateTime>,

    /// Whether the attachment appears inline in the item body.
    ///
    /// See <https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/isinline>
    pub is_inline: Option<bool>,
}