3rd party blocks

[Geal]

the work on 3rd party blocks has been advancing well, not finished yet but already enough to get started, so it is a good time to look at the Java implementation. The Rust and Haskell versions implement it and we will soon have a thorough serie of samples to work with.

3rd party blocks can be implemented first through multiple independent tasks:

• parsing an serializing scopes in protobuf
• verifying and creating external signatures, used in 3rd party blocks
• recognizing the trusting token in the Datalog parser
• refactoring the Datalog engine to support fact origins

Once all of those are done, the remaining work to get the full 3rd party blocks implementation will be straightforward. And i the meantime we will have finished specifying the v3 entirely. @divarvel and I can advise on the implementation if anyone wants to get started