Support selection of nested value within query result

[evilrobot-01]

• adds support for selection of nested value within query result, auto-converting balance values accordingly (bigint)
  • isolates change to a single function
• added simple test to ensure functionality works as expected, useful for regression testing. e.g. yarn zombienet-test -t tests -c tests/config.toml
• updated interface in readme

Example usage (from test.yml):

              - queries:
                  balance:
                    chain: *relay_chain
                    pallet: system
                    call: account
                    args: [ *account ]
                    selector: 'data.free'
              - asserts:
                  equal:
                    args: [ $balance, 1000000000000000000 ]

[evilrobot-01]

This might be improved by taking an array of selectors instead of a single value, which could then collect multiple nested selected values from a query result into an array, provided indexing the array could be supported in the assert yaml:

              - asserts:
                  - equal:
                       args: [ $result[0], true ]
                  - equal:
                       args: [ $result[1], 1000 ]

The basic idea is allowing the checking of certain attributes of a query result against expected values, rather than having to deal with the whole result.

Thinking out loud, maybe an even better approach is to support nested property syntax, where args are parsed and anything after the initial . is just passed to https://lodash.com/docs/#get to resolve the value. I'd be happy to implement if you think that it would ultimately be better than the current PR.

              - asserts:
                  - equal:
                       args: [ $result.booleanValue, true ]
                  - equal:
                       args: [ $result.numericValue, 1000 ]

[evilrobot-01]

Replaced with #79