Get Blockchain Nethash from API to allow sending transactions on testnet custom node

[slaweet]

Expected behaviour

Lisk Nano should get the nethash from /api/blocks/getNethash when user logs in into a Custom node, because there we use Mainnet nethash, but it could be also a testnet nethash.

Actual behaviour

Lisk Nano has hadrcoded the nethash for localhost development
https://github.com/LiskHQ/lisk-nano/blob/aa38811b75f310beabad3c69dfe6ad4dfd5b4213/src/components/login/networks.js#L18-L21

lisk-js is hardcoding the nethash for Mainnet and Testnet, so that's taken care of, but Lisk Nano doesn't handle nethash for Custom Node.

[slaweet]

activePeerSet action is a place to look at.