Pass all gas parameter in `ExternalCallContext`

[iFrostizz]

It is currently a bit unpractical to supply a constant max_units because some programs called externally may use more than others, and if we offload this to the parameters of the function then it adds more complexity. It would be nice to be able to pass an option to the current max_units field of the ExternalCallContext and in the case of this value being None, any unit that has not been spent would be refunded to the current context.

[richardpringle]

We can probably use an Option<NonZeroU64> and just treat it as a u64 on the go side. If the value is 0, that means to take what's needed.

[richardpringle]

Instead of passing in an Option<NonZeroU64> directly, I think it would be better to use a builder pattern for both Gas and Value.

ctx
    .call_contract_builder()
    .max_fuel(42u64)
    .transfer_value(1_000_000u64)
    .call_function("<function_name>", args)?;

And we could keep

ctx.call_function("<function_name>", args)?;

as the default usage when you don't need to provide a limited amount of gas or value

[iFrostizz]

When using constants, it's clear that passing 0 to forward all gas makes sense because the current behaviour would be not being able to execute anything but there could be footguns if the gas is dynamic and it would be harder to reason about when all the gas is forwarded vs what is passed. Currently done with an enum #1417 which avoids this pitfall but the builder pattern looks better.