Prep for MS Teams plugin API (#25565)

* - columns added to ShareChannelRemotes: lastpostcreateat, lastpostupdateat
- SyncMsg and SyncResponse moved to `model` package
- field added to RemoteCluster struct: PluginID

* sync new posts before updated posts to ensure post order in MS Teams

* add plugid to remoteclusters table and store

* don't sync history by default

server/public/model/shared_channel.go

@@ -4,6 +4,7 @@
 package model
 
 import (
+	"encoding/json"
 	"net/http"
 	"unicode/utf8"
 )
@@ -100,7 +101,9 @@ type SharedChannelRemote struct {
 	IsInviteConfirmed bool   `json:"is_invite_confirmed"`
 	RemoteId          string `json:"remote_id"`
 	LastPostUpdateAt  int64  `json:"last_post_update_at"`
-	LastPostId        string `json:"last_post_id"`
+	LastPostUpdateID  string `json:"last_post_id"`
+	LastPostCreateAt  int64  `json:"last_post_create_at"`
+	LastPostCreateID  string `json:"last_post_create_id"`
 }
 
 func (sc *SharedChannelRemote) IsValid() *AppError {
@@ -248,3 +251,49 @@ type SharedChannelRemoteFilterOpts struct {
 	RemoteId        string
 	InclUnconfirmed bool
 }
+
+// SyncMsg represents a change in content (post add/edit/delete, reaction add/remove, users).
+// It is sent to remote clusters as the payload of a `RemoteClusterMsg`.
+type SyncMsg struct {
+	Id        string           `json:"id"`
+	ChannelId string           `json:"channel_id"`
+	Users     map[string]*User `json:"users,omitempty"`
+	Posts     []*Post          `json:"posts,omitempty"`
+	Reactions []*Reaction      `json:"reactions,omitempty"`
+}
+
+func NewSyncMsg(channelID string) *SyncMsg {
+	return &SyncMsg{
+		Id:        NewId(),
+		ChannelId: channelID,
+	}
+}
+
+func (sm *SyncMsg) ToJSON() ([]byte, error) {
+	b, err := json.Marshal(sm)
+	if err != nil {
+		return nil, err
+	}
+	return b, nil
+}
+
+func (sm *SyncMsg) String() string {
+	json, err := sm.ToJSON()
+	if err != nil {
+		return ""
+	}
+	return string(json)
+}
+
+// SyncResponse represents the response to a synchronization event
+type SyncResponse struct {
+	UsersLastUpdateAt int64    `json:"users_last_update_at"`
+	UserErrors        []string `json:"user_errors"`
+	UsersSyncd        []string `json:"users_syncd"`
+
+	PostsLastUpdateAt int64    `json:"posts_last_update_at"`
+	PostErrors        []string `json:"post_errors"`
+
+	ReactionsLastUpdateAt int64    `json:"reactions_last_update_at"`
+	ReactionErrors        []string `json:"reaction_errors"`
+}