[Refactor] Serve certificates

[zjkmxy]

https://github.com/zjkmxy/ndn-workspace-solid/blob/dbb3c470b1fdc62c2a52a3cc78889a64686ebdd8/src/backend/main.ts#L99
https://github.com/zjkmxy/ndn-workspace-solid/blob/3e46cb7757ff9fcdaeabd339c16a4212d175e01a/src/backend/sync-agent/sync-agent.ts#L270

Now, all Data packet are served by SyncAgent.
Actually even CertStorage is not a real storage. (TODO: rename it to something else)
We need to either (1) separate responder out of SyncAgent or (2) officially support certificates in SyncAgent by adding a new channel.
Personally prefer (2). Discussion needed.