Adds support for GMs in shared channels (#31403)

* Adds support for GMs in shared channels

* Fix linter

* Remove creatorID from slack call

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Co-authored-by: Miguel de la Cruz <miguel@ctrlz.es>

server/platform/services/sharedchannel/service.go

@@ -53,6 +53,7 @@ type AppIface interface {
 	SendEphemeralPost(c request.CTX, userId string, post *model.Post) *model.Post
 	CreateChannelWithUser(c request.CTX, channel *model.Channel, userId string) (*model.Channel, *model.AppError)
 	GetOrCreateDirectChannel(c request.CTX, userId, otherUserId string, channelOptions ...model.ChannelOption) (*model.Channel, *model.AppError)
+	CreateGroupChannel(c request.CTX, userIDs []string, creatorId string, channelOptions ...model.ChannelOption) (*model.Channel, *model.AppError)
 	UserCanSeeOtherUser(c request.CTX, userID string, otherUserId string) (bool, *model.AppError)
 	AddUserToChannel(c request.CTX, user *model.User, channel *model.Channel, skipTeamMemberIntegrityCheck bool) (*model.ChannelMember, *model.AppError)
 	AddUserToTeamByTeamId(c request.CTX, teamId string, user *model.User) *model.AppError