implement balance_transfer_keep_alive

[brenzi]

currently, only allow_deathis supported. Even if we could compose our own extrinsics as we like, keep_alive would be a very common default way to transfer

substrate-api-client/src/extrinsic/balances.rs

Lines 42 to 62 in 8efd549

	pub trait BalancesExtrinsics {
	type Balance;
	type Address;
	type Extrinsic<Call>;
	/// Transfer some liquid free balance to another account.
	#[allow(clippy::type_complexity)]
	async fn balance_transfer_allow_death(
	&self,
	to: Self::Address,
	amount: Self::Balance,
	) -> Option<Self::Extrinsic<TransferAllowDeathCall<Self::Address, Self::Balance>>>;
	/// Set the balances of a given account.
	#[allow(clippy::type_complexity)]
	async fn balance_force_set_balance(
	&self,
	who: Self::Address,
	free_balance: Self::Balance,
	) -> Option<Self::Extrinsic<ForceSetBalanceCall<Self::Address, Self::Balance>>>;
	}