Consider making BlobRefRegistry a library

[ggonzalez94]

One of the ideas of the BlobRefRegistry contract was to serve as a common piece of infrastructure where blob hashes are validated and registered. If used by multiple rollups, this can save some gas and simplify their code. But whoever calls registerRef has to pay the SSTORE cost. Because of that, in our current version of the rollup we are not using this functionality, we are directly calling getRef to validate the blobs and return a reference.
If this continues to be the case, then it maybe makes more sense to simplify the implementation and convert the functionalities we are using into a library or just embed the logic into the contracts

[ggonzalez94]

We are now actually using the register function in the DelayedInclusionStore, but what we actually need is only the event to get emitted, so we might still want to simplify this(the cost for paying for storage is not necessary unless multiple rollups actually leverage that)