Support passing contract invocation args as JSON

[leighmcculloch]

This idea was shared or formed from some combination of @janewang @chadoh @willemneal (sorry I missed who put this in the doc but I think it was one of you!). I'm moving it here because the doc was focused on capturing the status quo.

Currently invoking a contract has the following format. It is elegant and meets most needs:

soroban contract invoke --id C123… -- do_something \
   --num 1 \
   --complex_arg '{ "field": 1 }'

There are situations where it may be helpful to form a single JSON blob for the args and pass them in as a single value either via an arg:

soroban contract invoke --id C123... \
   --args-json ‘{“method”: “do_something”, “args”: {“num”: 1, “complex_arg”: {“field”: 1}}’

or via stdin:

soroban contract invoke --id C123... -- do_something  << -
   {“num”: 1, “complex_arg”: {“field”: 1}}
-