Get more information about config by one HTTP interface

## Enhancement Task
Now, pd supports `/config` and `/config/default` interfaces to get configurations' current value and default value. I would like to propose one interface to get details of all configurations.

**Route Path**: `/config/detail`
**Method**: GET
**Response Body**: []ConfigDetail
```
// ConfigDetail is the detail of config option with current value and default value.
type ConfigDetail struct {
// config name
Name string `json:"name"`
// current config value
Value any `json:"value"`
// default config value
DefaultValue any `json:"default_value"`
}
```
What's more, we can then refine the interface by adding new fields to the structure, such as maximum, minimum, type, enumeration, and so on.

Start by locating the existing `/config` and `/config/default` HTTP handlers and their tests. Add the `/config/detail` GET endpoint so it returns a `[]ConfigDetail` containing each configuration's name, current value, and default value; done means the response matches the specified JSON shape.

Written by the indexing model from the issue text.