MM-54023: [Shared Channels] Filter out system posts for channel update info that is not synced (#30735)

* initial checkin

* simplify tests, logic

* fix tests

* remove unneeded test

* fix spacing

* updates

* update logic

* fix logic

* move filtering to DB

* remove comment

---------

Co-authored-by: Catalin Tomai <catalintomai@catalins-macbook-pro-2.home>

server/public/model/post.go

@@ -386,10 +386,11 @@ func (c GetPostsSinceForSyncCursor) IsEmpty() bool {
 }
 
 type GetPostsSinceForSyncOptions struct {
-	ChannelId       string
-	ExcludeRemoteId string
-	IncludeDeleted  bool
-	SinceCreateAt   bool // determines whether the cursor will be based on CreateAt or UpdateAt
+	ChannelId                         string
+	ExcludeRemoteId                   string
+	IncludeDeleted                    bool
+	SinceCreateAt                     bool // determines whether the cursor will be based on CreateAt or UpdateAt
+	ExcludeChannelMetadataSystemPosts bool // if true, exclude channel metadata system posts (header, display name, purpose changes)
 }
 
 type GetPostsOptions struct {