Improve NeoFS BlockFetcher/BlockUploader object attributes

[AnnaShaleva]

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

Existing NeoFS containers that store Neo chains have two problems:

1. Object attributes start from lowercase letters: block:0, index:0.
2. Both block and index file objects don't have network magic included into attributes, and hence it's impossible to learn the network of the block or index file without additional external knowledge like this:

neo-testnet container: HmP48JvCcmiEbJemG7jFNEg2qkdCTx4thD3EwWUi288Q
neo-mainnet container: 2W2c7qHqicZJJdRG134k7eusZqidkk3KBDNgeLL45nn2
fschain testnet 4SJK36y4JGZJ5rjXsGWsJ3yiWrcvEANXBBHThQ6eErsq
fschain mainnet 5oAvDsnVcpGEf8KM4nxNB52wqZASjbr4CSszbW9iseCk

Describe the solution you'd like

Reuploading currently used containers doesn't make sense, but for neo-project/neo#3463 it should be done:

1. Capitalize block objects and index file objects attributes.
2. Add network magic attribute to both block and index file objects.

Describe alternatives you've considered

Leave it as is, it also works.

[AnnaShaleva]

We have to reupload existing chains anyway since there are a lot of corrupted blocks/index files due to script bugs, hence we may improve format right now.