Support `Credential()` method 

The method should return a `Credential()` function that can be used by [auth.Client](https://pkg.go.dev/oras.land/oras-go/v2@v2.0.2/registry/remote/auth#Client) of oras-go v2.

Proposed API:
```go
package credentials

func Credential(store Store) func(context.Context, string) (auth.Credential, error) {
}
```

Design discussion: #18 