MM-43145: channels loader (#19957)

Add channels dataloader

https://mattermost.atlassian.net/browse/MM-43145

```release-note
NONE
```

store/storetest/channel_store.go

@@ -72,6 +72,7 @@ func TestChannelStore(t *testing.T, ss store.Store, s SqlStore) {
 	t.Run("Update", func(t *testing.T) { testChannelStoreUpdate(t, ss) })
 	t.Run("GetChannelUnread", func(t *testing.T) { testGetChannelUnread(t, ss) })
 	t.Run("Get", func(t *testing.T) { testChannelStoreGet(t, ss, s) })
+	t.Run("GetMany", func(t *testing.T) { testChannelStoreGetMany(t, ss, s) })
 	t.Run("GetChannelsByIds", func(t *testing.T) { testChannelStoreGetChannelsByIds(t, ss) })
 	t.Run("GetChannelsWithTeamDataByIds", func(t *testing.T) { testGetChannelsWithTeamDataByIds(t, ss) })
 	t.Run("GetForPost", func(t *testing.T) { testChannelStoreGetForPost(t, ss) })
@@ -476,6 +477,40 @@ func testChannelStoreGet(t *testing.T, ss store.Store, s SqlStore) {
 	s.GetMasterX().Exec("TRUNCATE Channels")
 }
 
+func testChannelStoreGetMany(t *testing.T, ss store.Store, s SqlStore) {
+	o1, nErr := ss.Channel().Save(&model.Channel{
+		TeamId:      model.NewId(),
+		DisplayName: "Name",
+		Name:        NewTestId(),
+		Type:        model.ChannelTypeOpen,
+	}, -1)
+	require.NoError(t, nErr)
+
+	o2, nErr := ss.Channel().Save(&model.Channel{
+		TeamId:      model.NewId(),
+		DisplayName: "Name2",
+		Name:        NewTestId(),
+		Type:        model.ChannelTypeOpen,
+	}, -1)
+	require.NoError(t, nErr)
+
+	res, err := ss.Channel().GetMany([]string{o1.Id, o2.Id}, true)
+	require.NoError(t, err)
+	assert.Len(t, res, 2)
+
+	res, err = ss.Channel().GetMany([]string{o1.Id, "notexists"}, true)
+	require.NoError(t, err)
+	assert.Len(t, res, 1)
+
+	_, err = ss.Channel().GetMany([]string{"notexists"}, true)
+	require.Error(t, err)
+	var nfErr *store.ErrNotFound
+	require.True(t, errors.As(err, &nfErr))
+
+	// Manually truncate Channels table until testlib can handle cleanups
+	s.GetMasterX().Exec("TRUNCATE Channels")
+}
+
 func testChannelStoreGetChannelsByIds(t *testing.T, ss store.Store) {
 	o1 := model.Channel{}
 	o1.TeamId = model.NewId()

store/storetest/mocks/ChannelStore.go

@@ -925,29 +925,6 @@ func (_m *ChannelStore) GetForPost(postID string) (*model.Channel, error) {
 	return r0, r1
 }
 
-// GetFromMaster provides a mock function with given fields: id
-func (_m *ChannelStore) GetFromMaster(id string) (*model.Channel, error) {
-	ret := _m.Called(id)
-
-	var r0 *model.Channel
-	if rf, ok := ret.Get(0).(func(string) *model.Channel); ok {
-		r0 = rf(id)
-	} else {
-		if ret.Get(0) != nil {
-			r0 = ret.Get(0).(*model.Channel)
-		}
-	}
-
-	var r1 error
-	if rf, ok := ret.Get(1).(func(string) error); ok {
-		r1 = rf(id)
-	} else {
-		r1 = ret.Error(1)
-	}
-
-	return r0, r1
-}
-
 // GetGuestCount provides a mock function with given fields: channelID, allowFromCache
 func (_m *ChannelStore) GetGuestCount(channelID string, allowFromCache bool) (int64, error) {
 	ret := _m.Called(channelID, allowFromCache)
@@ -969,6 +946,29 @@ func (_m *ChannelStore) GetGuestCount(channelID string, allowFromCache bool) (in
 	return r0, r1
 }
 
+// GetMany provides a mock function with given fields: ids, allowFromCache
+func (_m *ChannelStore) GetMany(ids []string, allowFromCache bool) (model.ChannelList, error) {
+	ret := _m.Called(ids, allowFromCache)
+
+	var r0 model.ChannelList
+	if rf, ok := ret.Get(0).(func([]string, bool) model.ChannelList); ok {
+		r0 = rf(ids, allowFromCache)
+	} else {
+		if ret.Get(0) != nil {
+			r0 = ret.Get(0).(model.ChannelList)
+		}
+	}
+
+	var r1 error
+	if rf, ok := ret.Get(1).(func([]string, bool) error); ok {
+		r1 = rf(ids, allowFromCache)
+	} else {
+		r1 = ret.Error(1)
+	}
+
+	return r0, r1
+}
+
 // GetMember provides a mock function with given fields: ctx, channelID, userID
 func (_m *ChannelStore) GetMember(ctx context.Context, channelID string, userID string) (*model.ChannelMember, error) {
 	ret := _m.Called(ctx, channelID, userID)