Add support for: (i) Proposal's Transient and (ii) Idemix

Currently, FPC does not support the following:

- `Proposal's Transient field` that is used to pass ephemeral information to the chaincode. This is useful in many settings where some piece of information is required by the computation needed by the chaincode but it should not appear on the ledger in any form.
- `Idemix` signatures. Idemix is an anonymous credential system integrated in Fabric that allows users to sign anonymously proposals and transactions. It is useful in any context where the submitter's identity must be concealed.