[sync] HTTP client for downloading remote transactions

[grigoryk]

Client will be written around the API described in https://github.com/rfk/mentat-sync-prototype/

General flow:

    Using `SyncMetadataClient`, get the currently known remote_head.
    Get list of new transactions via GET /transactions?from={remote_head}
    For each transaction:
        Get list of chunks via GET /transactions/{trn}
        For each chunk:
            Get chunk contents via GET /chunks/{chunk}
    Return a list of transactions w/ chunks

[grigoryk]

Completed in #563.