allow padding of GAS / specifying GAS amount for invoke TX

[ixje]

There are use-cases in which you would like to attach more GAS than required to just execute the main code path or exactly specify the attached GAS. This can be for example to allow the contract to burn the remaining GAS to the benefit of the NEO community (here is an example of such a contract that does this), or simply to debug insufficient GAS scenario's.

At this time all invocations seem to default to 20 GAS (as defined by TestModeGas) https://github.com/neo-project/neo-debugger/blob/1194d55ed85d9f0a7ff821700ae0ef718a864f6e/src/adapter3/DebugApplicationEngine.cs#L30

[ixje]

Another real use-case is when you want to test/debug the fixing of storage on a contract and the cost of that exceeds 20 GAS.