Investigate bytecode metadata and service APIs for tooling around transactions

[ma2bd]

Block explorers should be able to make sense of transactions (operations and messages) in blocks.
However, the (presumably BCS-based) binary format is application-specific.

One seemingly easy solution would be to add APIs to the service bytecode for JSON<>BCS conversions.

We should also let bytecode authors share additional information when they upload their code. Notably, the URL of the source code. Eventually reproducible builds could be checked by the community.