Ensure stability of the structure of block and shard serialization

[frol]

Even before we get to the refactoring of near-indexer StreamerMessage struct, we need to have at least a minimum safe-check that the data that we push to S3 has the same layout as it is scary to upgrade to the new nearcore releases without such checks. It might be actually part of nearcore test suite for near-indexer: grab some complex block with many different transactions, receipts, execution outcomes and shards and check that we can serialize it to the same JSON string.