Make marker datum customisable

[ghost]

Why

• Fuelling wallet internal to the hydra-node requires creation of a UTxO with a marker datum which the wallet can then pick and use to pay for transactions fees
• This marker datum is hardcoded hence is the same for everyone
• Extracting the value of this datum from the code is a bit annoying, which can hinder efforts to script the deployment of nodes and creation of heads

What

Make the marker datum hash configurable when starting the hydra-node. This could be achieved in a myriad ways: CLI arguments, env variable, giving a string and then hashing internally...

[ch1bo]

To clarify: You suggest we make the x bytes as hex string configurable and use that instead of markerDatumHash in our code.

Will we need the pre-image of this datum hash when spending fuel UTxO?

[ch1bo]

When we do #215, this becomes irrelevant.

[v0d1ch]

This became redundant with version 0.12