Stateless chain observation + tool

[ghost]

Why

To enable easy implementation of explorers, alternative implementations of the hydra-node it would be beneficial if any transaction itself (along with it's inputs) can be determined to be a Hydra protocol transaction.

• Design work has already been done and proposed as an ADR along with some experimentations
• This feature will simplify the internals of the Direct chain component and unlock other features on our roadmap or backlog:
  • Log and send client output when some keys don't match #529
  • Build and deploy a Hydra heads explorer #696
  • Incremental decommit #1057
  • Incremental commit #199

This capability can be demonstrated by an executable connecting to the chain and just outputting all Hydra transactions.

What

• An executable (or subcommand) of the hydra-node package connects to the chain and outputs all valid hydra protocol transactions as JSON on stdout, e.g. named hydra-chain-observer.
  • Notably, this executable should not need any head parameters or keys (to find "the right head", all heads are observed)
• Direct chain observation code carries a lot of internal state: Get rid of it so that we can observe on-chain state of a head only by following the chain

How

• Observe txs without having to know nothing more than some static information, eg. the scripts' addresses and the "standard" token name
• This implies someone might try to trick us as it's easy to name a token HydraHeadV1?
• Each observation is keyed by at the headId it relates to which is inferred from the InitTx

Relevant previous work:

• https://github.com/input-output-hk/hydra/tree/abailly-iohk/head-observer
• https://github.com/input-output-hk/hydra/compare/ch1bo/stateless-hack

• Implement a simple stateless observation executable #1158
• Simplify observation / construction using inline datums Protocol change: Inline datums #1162
• Utilize observeHeadTx in hydra-node and only keep track of UTxO (like the hydra-chain-observer) Stateless observation hydra node #1196