Add command to create Genesis keypair in aggregator

[jpraynaud]

Why

When setting up a test aggregator we may need to create test genesis keys and there is no command available on the aggregator for doing this.

What

Add a new sub-command to the genesis command to create the non deterministic genesis keypair.

How

• Create a new sub-command generate-keypair for the genesis command
  • by using the create_non_deterministic_genesis_signer (see test_codec_genesis_keypair test)
  • with a custom export directory
  • by exporting a genesis.sk and genesis.vk files in the export directory
• Update documentation