Add entity level digital signing

[gra-moore]

There is a big need for enhanced trust on data. One way to help with this is to digitally signed data that is received and stored and then subsequently published. The data would be signed using the datahub or an additional signing private key. The datahub would make the public key for these available, and the entity it self would retain the signature. A client would be able to verify that data had not been manipulated at any step in the process.

While this sounds a bit block-chainy. It is not. For many of these cases we need to be able to trust the data, we are not so concerned by the chains of events or the ledger aspects of things. Indeed we want a distributed model of trust.

There will need to be some good patterns where an entity could point to a source entity as part of a series of transforms, or perhaps the original data is kept as part of a transformed entity.