Adds support for GMs in shared channels (#31403)

* Adds support for GMs in shared channels

* Fix linter

* Remove creatorID from slack call

---------

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

server/platform/services/sharedchannel/mock_AppIface_test.go

@@ -102,6 +102,45 @@ func (_m *MockAppIface) CreateChannelWithUser(c request.CTX, channel *model.Chan
 	return r0, r1
 }
 
+// CreateGroupChannel provides a mock function with given fields: c, userIDs, creatorId, channelOptions
+func (_m *MockAppIface) CreateGroupChannel(c request.CTX, userIDs []string, creatorId string, channelOptions ...model.ChannelOption) (*model.Channel, *model.AppError) {
+	_va := make([]interface{}, len(channelOptions))
+	for _i := range channelOptions {
+		_va[_i] = channelOptions[_i]
+	}
+	var _ca []interface{}
+	_ca = append(_ca, c, userIDs, creatorId)
+	_ca = append(_ca, _va...)
+	ret := _m.Called(_ca...)
+
+	if len(ret) == 0 {
+		panic("no return value specified for CreateGroupChannel")
+	}
+
+	var r0 *model.Channel
+	var r1 *model.AppError
+	if rf, ok := ret.Get(0).(func(request.CTX, []string, string, ...model.ChannelOption) (*model.Channel, *model.AppError)); ok {
+		return rf(c, userIDs, creatorId, channelOptions...)
+	}
+	if rf, ok := ret.Get(0).(func(request.CTX, []string, string, ...model.ChannelOption) *model.Channel); ok {
+		r0 = rf(c, userIDs, creatorId, channelOptions...)
+	} else {
+		if ret.Get(0) != nil {
+			r0 = ret.Get(0).(*model.Channel)
+		}
+	}
+
+	if rf, ok := ret.Get(1).(func(request.CTX, []string, string, ...model.ChannelOption) *model.AppError); ok {
+		r1 = rf(c, userIDs, creatorId, channelOptions...)
+	} else {
+		if ret.Get(1) != nil {
+			r1 = ret.Get(1).(*model.AppError)
+		}
+	}
+
+	return r0, r1
+}
+
 // CreatePost provides a mock function with given fields: c, post, channel, flags
 func (_m *MockAppIface) CreatePost(c request.CTX, post *model.Post, channel *model.Channel, flags model.CreatePostFlags) (*model.Post, *model.AppError) {
 	ret := _m.Called(c, post, channel, flags)