Support custom session length in config.json

[JoshOrndorff]

Since paritytech/polkadot#2167, polkadot has supported a custom session length in the chain spec file. However this property is not part of the runtime_genesis_config, which means it can't be modified by the mechanism introduced in #88.

This issue is to support custom session length in polkadot-launch's config.json. I can imagine two potential solutions.

1. Extend polkadot-launch to support it
2. Modify polkadot so it goes in the normal genesis config (but I'm guessing there is a good reason it wasn't put there to begin with?).

[shawntabrizi]

@JoshOrndorff i think it should be easy enough to support it, but can you describe how to do it.

It seems you want to target an object that is one level higher than the hook i wrote for updating config?

[JoshOrndorff]

Yeah exactly. The output of polkadot build-spec --chain rococo-local is (of the form)

{
  "name": "Rococo Local Testnet",
  "id": "rococo_local_testnet",
  "chainType": "Local",
  "bootNodes": [],
  "telemetryEndpoints": null,
  "protocolId": "dot",
  "properties": null,
  "forkBlocks": null,
  "badBlocks": null,
  "consensusEngine": null,
  "lightSyncState": null,
  "genesis": {
    "runtime": {
      "runtime_genesis_config": {
        // Content elided
        // All of this stuff is already handled properly
      },
      // This is the different one that I'd like to set
      "session_length_in_blocks": 10
    }
  }
}