chore(polkadot): upgrade to v1.2.0

[renauter]

Description

• [x] Upgrade to Polkadot v1.2.0
  • [x] update versions of clap and serde_json dependencies
  • [x] add block_relay to sc_service::build_network()
  • [x] pallet-smart-contract: update Balances::transfer() function for tests
  • [x] add sp-genesis-builder dependency
  • [x] implement sp_genesis_builder::GenesisBuilder<Block> for runtime
  • [x] pallet_balances: prefer using transfer_keep_alive() instead of transfer()

Related Issues:

• Closes #906

Checklist:

Please delete options that are not relevant.

• [x] My commits follow this conventional commits guide.