Support WritePack in OdbBackend

In order to properly implement a backend, you need to be able to let a fetching process write data into the packfile. While GitOdbBackend has this field, OdbBackend does not expose this method, which means that a repository using only a managed backend cannot be full-featured.

/cc @synhershko @nulltoken

首先，将 GitOdbBackend 中现有的 WritePack 字段与 OdbBackend 进行比较，并追踪 managed backends 的使用方式。确认 fetching 进程写入 packfile 数据所需的 backend API，然后验证仅使用 managed backend 的 repository 能够具备完整功能。