Add support for chain spec modifier commands

[Fahrrader]

In response to https://github.com/paritytech/zombienet/issues/892.

[cla-bot-2021[bot]]

User @Fahrrader, please sign the CLA here.

[Fahrrader]

Tests (and examples and docs, probably) are required, but putting the draft out there for review :)

[wirednkod]

@Fahrrader , also, please run lint prior to next push, for the checks to pass

[Fahrrader]

Docs, examples and tests (hopefully) are up and ready!

[Fahrrader]

@pepoviola @wirednkod I think this PR is good to go and ready for the final review! I believe all the previous concerns were addressed. Please take another look, if not already.

[pepoviola]

Hi @Fahrrader, sorry about the delay. First let me say thanks for contribution and your feedback. We had discussed with the team and some security concerns arise from allow users to execute arbitrary 3rd party tools as part of the spawning process. We have a couple of options, we can built-in some kind of customization tool that allow us to have more control of what is executed, we can allow this execution (giving us more flexibility) and add some guards (like don't allow to run with root access) or we can also make the spawning process more flexible and add a command to just produce the chain artifacts so allow others tools to interact with zombienet to their needs.

I will a new discussion in the repo to collect some feedback before go into one of the paths. Agains, sorry about the delay and thanks for your contribution and feedback!!

[Polkadot-Forum]

This pull request has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/discover-chainql-unique-networks-advanced-tool-for-substrate-chain-storage/2861/1