Generate Builder Pattern for external contract representations

[swfsql]

There are some discussions related to this, such as:

• near.zulip
• (add more)

External Contract's methods can be represented by a builder, something like:

_impl::Request::contract("my.contract".parse().unwrap())
        .args(String::from("my_value"))
        .send_amount(0)
        .prepaid_gas(Gas::from(SINGLE_CALL_GAS))
        .request();

Or a current code example.

They must be able to be built client-side, such as from the trait definitions as it is today. For when the client has access into the external contract's src code, this builder can be generated by the external contract's src code itself, so the client would only need to import and then use the builder.

For builders prepared on specific impls, they can take advantage of the payable, or maybe gas = x attributes to eg. actually refrain from implementing payment option, or make a attached gas check to early panic (instead of making a call and then it panicking on the server contract)

Another related feature is to have a generic external contract call while also having a builder-pattern structure. This is mostly implemented in here (outdated) but it could use more design iteration.