delegator build_call example

[Daanvdplas]

I wrote a different implementation of the delegator example. In stead of instantiating all the contracts when instantiating the delegator contract, the delegator contract is instantiated with already instantiated contracts of adder, subber and accumulator.

Why I think it could be useful:

• It shows a simple but extended example with multiple contracts interacting with each other through build_call.
• Reference material to compare with the other delegator (now called multi_contract_caller example).
• Supported by e2e-tests.

Please let me know if it needs any improvements (e.g. the name 😅)!

[SkymanOne]

Closing this PR as #1826 and #1825 need to be addressed first