(Investigate) Move call definition into extrinsic traits

[haerdib]

According to #434 (comment):

I'm wondering if we couldn't also "move" these types into the trait?
Do you think it's overkill or is there a specific reason not to do it?

One should look into if it makes sense to move the call definitions

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

Lines 33 to 38 in e27d177

	/// Call for a balance transfer.
	pub type TransferCall<Address, Balance> = (CallIndex, Address, Compact<Balance>);
	/// Call to the balance of an account.
	pub type SetBalanceCall<Address, Balance> =
	(CallIndex, Address, Compact<Balance>, Compact<Balance>);

as a trait type. Should be a simplification, not something that makes things more compilacated.