Fix all possible setting error related storages and added some tests (#23911)

Follow up #22405

Fix #20703 

This PR rewrites storage configuration read sequences with some breaks
and tests. It becomes more strict than before and also fixed some
inherit problems.

- Move storage's MinioConfig struct into setting, so after the
configuration loading, the values will be stored into the struct but not
still on some section.
- All storages configurations should be stored on one section,
configuration items cannot be overrided by multiple sections. The
prioioty of configuration is `[attachment]` > `[storage.attachments]` |
`[storage.customized]` > `[storage]` > `default`
- For extra override configuration items, currently are `SERVE_DIRECT`,
`MINIO_BASE_PATH`, `MINIO_BUCKET`, which could be configured in another
section. The prioioty of the override configuration is `[attachment]` >
`[storage.attachments]` > `default`.
- Add more tests for storages configurations.
- Update the storage documentations.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/lfs/server.go

@@ -452,7 +452,7 @@ func buildObjectResponse(rc *requestContext, pointer lfs_module.Pointer, downloa
 
 		if download {
 			var link *lfs_module.Link
-			if setting.LFS.ServeDirect {
+			if setting.LFS.Storage.MinioConfig.ServeDirect {
 				// If we have a signed url (S3, object storage), redirect to this directly.
 				u, err := storage.LFS.URL(pointer.RelativePath(), pointer.Oid)
 				if u != nil && err == nil {