Derive hydra-scripts-tx-id from network parameters if no explicit value is provided

[errfrom]

Why

This would simplify configuration for users who trust the claim that the Hydra scripts are published in unspendable utxos and don't want to publish the scripts themselves.

What

--hydra-scripts-tx-id CLI parameter could accept an explicit default value indicating that the user wants to use pre-published scripts.

How

The scripts could be published before release, and the resultant transaction ids would then be "encoded" in hydra-node and selected based on the network.