Implement experimental fetching of blocks from NeoFS container

[roman-khimov]

Is your feature request related to a problem? Please describe.

I'm always frustrated when we're fetching blocks via P2P. They can be stored in NeoFS easily.

Describe the solution you'd like

We don't yet have any proper solution for block uploads, but this shouldn't stop us experimenting with formats and fetching. Upload testnet blocks to testnet and arrange an option to fetch them, see how it goes. We need to standardize on attributes, index/primary index/block hash/prev hash/timestamp are obvious candidates to be stored in object attributes. We will also need to store blocks and headers in different objects (to be able to do different requests). Hash indexes are also needed (2000 hashes in each with some attributes to find them). Some blockdump-like chunks of data (1-2K of blocks) can also be thought of, although not strictly required now.