feat(client) add --starting-height flag to simd init

[fragwuerdig]

Summary

I propose to add a flag to the simd init command, that controls the default starting block height of the output genesis.json

Problem Definition and Proposal

I am frequently spinning local testnets with multiple validators for the blockchain that I am working on. On softforks of the codebase the behavior of these testnets happen to be block height dependent. This comes with the extra burden of creating tooling around modifying the genesis.json after simd init. It feature would not add any "value" to Cosmos SDK. It's just a neat feature. Nice to have. And I am totally happy to implement this on my own. So, please, assign to me.

[alexanderbez]

For sure @fragwuerdig, I think this is a small value add w/o any significant tech overhead. Happy to review a PR!

[tac0turtle]

Thanks for opening this issue, we can accommodate this feature. It makes me think if we should add a genesis modification tool to the tools directory since many people have scripts