MM-21357: Use typed constant for channel types (#17928)

* MM-21357: Use typed constant for channel types

https://mattermost.atlassian.net/browse/MM-21357

```release-note
- Introduced a new type ChannelType for all channel types.
- Updated the Client4.UpdateChannelPrivacy method to ChannelType.
```

* Address review comments

```release-note
NONE
```

* telemetry fix

```release-note
NONE
```

model/shared_channel.go

@@ -16,19 +16,19 @@ import (
 // If "home" is false, then the shared channel is homed remotely, and "RemoteId"
 // field points to the remote cluster connection in "RemoteClusters" table.
 type SharedChannel struct {
-	ChannelId        string `json:"id"`
-	TeamId           string `json:"team_id"`
-	Home             bool   `json:"home"`
-	ReadOnly         bool   `json:"readonly"`
-	ShareName        string `json:"name"`
-	ShareDisplayName string `json:"display_name"`
-	SharePurpose     string `json:"purpose"`
-	ShareHeader      string `json:"header"`
-	CreatorId        string `json:"creator_id"`
-	CreateAt         int64  `json:"create_at"`
-	UpdateAt         int64  `json:"update_at"`
-	RemoteId         string `json:"remote_id,omitempty"` // if not "home"
-	Type             string `db:"-"`
+	ChannelId        string      `json:"id"`
+	TeamId           string      `json:"team_id"`
+	Home             bool        `json:"home"`
+	ReadOnly         bool        `json:"readonly"`
+	ShareName        string      `json:"name"`
+	ShareDisplayName string      `json:"display_name"`
+	SharePurpose     string      `json:"purpose"`
+	ShareHeader      string      `json:"header"`
+	CreatorId        string      `json:"creator_id"`
+	CreateAt         int64       `json:"create_at"`
+	UpdateAt         int64       `json:"update_at"`
+	RemoteId         string      `json:"remote_id,omitempty"` // if not "home"
+	Type             ChannelType `db:"-"`
 }
 
 func (sc *SharedChannel) ToJson() string {