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>

routers/web/base.go

@@ -19,11 +19,11 @@ import (
 	"code.gitea.io/gitea/modules/web/routing"
 )
 
-func storageHandler(storageSetting setting.Storage, prefix string, objStore storage.ObjectStorage) func(next http.Handler) http.Handler {
+func storageHandler(storageSetting *setting.Storage, prefix string, objStore storage.ObjectStorage) func(next http.Handler) http.Handler {
 	prefix = strings.Trim(prefix, "/")
 	funcInfo := routing.GetFuncInfo(storageHandler, prefix)
 	return func(next http.Handler) http.Handler {
-		if storageSetting.ServeDirect {
+		if storageSetting.MinioConfig.ServeDirect {
 			return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
 				if req.Method != "GET" && req.Method != "HEAD" {
 					next.ServeHTTP(w, req)