Versioning of `CheckpointPolicy` script

[marcinbugaj]

See here

And then there is the question of the TODO on line 151, which I'm not sure how to address. Note that:

• CheckpointParamater is an AssetClass, but we only ever mint a single NFT with the CurrencySymbol of that asset class. See here. We can therefore turn this asset class into a currency symbol, just like every other place that uses a currency symbol.

• CheckpointPolicy is currently not being versioned (see here), but the above bullet is an indicator that it probably should be versioned. See here.

• Implementing this recommendation will have a knock-on effect of making CheckpointParameter a newtype wrapper around SidechainParams, making it redundant. See my other comment on UpdateCommitteeHash.

[marcinbugaj]

https://input-output.atlassian.net/browse/ETCM-6609