Fix all possible setting error related storages and added some tests (#23911) (#25244)

Backport #23911 by @lunny

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

routers/web/repo/attachment.go

@@ -126,7 +126,7 @@ func ServeAttachment(ctx *context.Context, uuid string) {
 		return
 	}
 
-	if setting.Attachment.ServeDirect {
+	if setting.Attachment.Storage.MinioConfig.ServeDirect {
 		// If we have a signed url (S3, object storage), redirect to this directly.
 		u, err := storage.Attachments.URL(attach.RelativePath(), attach.Name)