override: Inject cores to produce blocks with elastic scaling

[lexnv]

This PR allows local testing with Elastic Scaling from custom parachains

Builds on top of:

• Add support for custom parachains #110

cc @pepoviola @sandreim

[pepoviola]

Hey @lexnv, looks great. I have a couple of questions:

• Can we assign cores per para individually, iiuc with the current code all paras get the same amount of cores?
• I think we should also work to make the max cores available dynamic, currently is capped by the predefined accounts for validators but that is a hardcoded value that we can remove.
• Currently Kusama and Polkadot have diff lookahead value, maybe we can change the override and lookahead value for each one.

Thanks!!