Add log format version to data

[swcurran]

This idea comes from our discussions in defining did:webvh (formerly did:tdw -- https://identity.foundation/trustdidweb/next). It would be good to have a version defined in the log that defines what version of this specification that the writer is using, and to allow a new version to be used later in the log to change the version. That enables:

• Defining a tightly as needed (possible) what dependent specifications are being used in a given version of the spec -- e.g. what hash algorithms are allowed, what proof formats are allowed, and so on. This simplifies implementations and enables interoperability.
• Updating the version allows the specification to evolve, and to support later dependent specifications -- such as post-quantum.
• Multibase, etc. should still be used to enable multiple (for example, hash, DI) algorithms to be used in a version, but should not be used as a reason to let any writer use anything they want. That's too much of a burden on log readers.
• By being able to change the version over time, the logs can be long-lasting, which we assume is a goal. In our case of DID Documents, we want them to last for many years. Being "agile" with the underlying algorithms is crucial.

[msporny]

Yes, this is an excellent point, I'll add a version flag to the spec.