[MM-59367] export: enable exporting thread followers for CRT (#27623)

server/channels/app/imports/import_types.go

@@ -187,6 +187,8 @@ type PostImportData struct {
 	Replies     *[]ReplyImportData      `json:"replies,omitempty"`
 	Attachments *[]AttachmentImportData `json:"attachments,omitempty"`
 	IsPinned    *bool                   `json:"is_pinned,omitempty"`
+
+	ThreadFollowers *[]ThreadFollowerImportData `json:"thread_followers,omitempty"`
 }
 
 type DirectChannelImportData struct {
@@ -213,6 +215,8 @@ type DirectPostImportData struct {
 	Replies     *[]ReplyImportData      `json:"replies"`
 	Attachments *[]AttachmentImportData `json:"attachments"`
 	IsPinned    *bool                   `json:"is_pinned,omitempty"`
+
+	ThreadFollowers *[]ThreadFollowerImportData `json:"thread_followers,omitempty"`
 }
 
 type SchemeImportData struct {
@@ -255,3 +259,11 @@ type ComparablePreference struct {
 	Category string
 	Name     string
 }
+
+type ThreadFollowerImportData struct {
+	// User is the username of the follower. It's the general convention
+	// for import data types to name it as user for the username.
+	User           *string `json:"user"`
+	LastViewed     *int64  `json:"last_viewed,omitempty"`
+	UnreadMentions *int64  `json:"unread_mentions,omitempty"`
+}