Implement Native Store

Design discussion: https://github.com/oras-project/oras-credentials-go/discussions/18

References:
- [Notation implementation](https://github.com/notaryproject/notation/blob/main/pkg/auth/native_store.go)
- [Docker CLI implementation](https://github.com/docker/cli/blob/v20.10.23/cli/config/credentials/native_store.go)
