Support for custom tx payload builder

[michalkucharczyk]

Added with_tx_payload_builder_sub and with_tx_payload_builder_eth methods to ScenarioBuilder, allowing users to provide custom transaction payload builders instead of using the built-in transfer/remark recipes. The custom builder receives a TxBuildContext with account
info and nonce, enabling arbitrary extrinsic calls.

An example (examples/kill_dev_entry.rs) demonstrates how to use new feature to build custom CLI sender. It uses by sending TestPallet::kill_dev_entry transactions across a range of accounts.

[iulianbarbu]

Looks very nice, thanks!

[iulianbarbu]

Hmm, we have a clippy error in relation to too many args for a fn. Probably fixable with some effort.

[michalkucharczyk]

It is a bit clumsy that we have two methods, but for now I don't have better idea how to handle this. Probably entire scenario builder should be generic over the config - the thing I wanted to avoid at the beginning. Something to think about as a side project maybe...

[iulianbarbu]

lgtm!

[iulianbarbu]

It is a bit clumsy that we have two methods,

You mean the two methods for tx builder payload, on the scenario builder?

[iulianbarbu]

Probably entire scenario builder should be generic over the config - the thing I wanted to avoid at the beginning. Something to think about as a side project maybe...

It is not necessarily a bad idea, it is a single generic type for now, unless you tried in the past and it turned out to be a mess.