Add a way to include signatures for genesis smith certificates

So far, we have only created clique smiths WoT to simplify the setup of test networks.
At some point, we should train ourselves to create a real genesis smith WoT based on who wants to be included and the smith certs they want to issue at genesis.

We will probably create a dedicated topic on the Duniter forum for candidates, but we should be able to verify that users' choices come from real Ğ1 identities—a forum account is not secure enough.

My proposal for this is to create a simple payload format to sign with gcli and verify during the genesis build.

Payload format: <currency name>|<iteration index>|<hexadecimal pubkey>

For example, if the account g1Q1EVXbtbD5pt1tWrD4XXjJJ2FVPGuLCkuWQRrvMCJVookBE wants to join the smith genesis, they should sign the string:
gtest|2|0xde4ae257be5494b63a4ba1582a6a8e7b2ece2cef54fddde98bab435b62c65b63.

If the account g1Q1EVXbtbD5pt1tWrD4XXjJJ2FVPGuLCkuWQRrvMCJVookBE wants to certify the account g1L1bpHMLShdSSAsc9WxQmVXL7umWNxXT9dPRqu2GdCe8yLjb, they should sign the string:
gtest|2|0x2da1bb2b95ecb78051e2b93f210327190004a9fd2f8af4d1f256645297585eaf.

The purpose of the iteration number is to ensure users can change their choices between iterations of the same currency.