Rootnet membership configuration on-chain

[adlrocha]

Related: #21
We use a config file to pass the membership information in Mir-based rootnets. For Spacenet and beyond this may be fine, but if in the future we want to run a PoS-like rootnet based on Mir, we'll need to push the membership information on-chain into its own "membership actor".

[adlrocha]

This has been implemented and tested here: https://github.com/consensus-shipyard/lotus/pull/117/files

Now subnets can pick up the membership from the membership information in their parent through the IPC agent.