Add an option for `WalletPlugin` to disallow any transaction modification

[aaroncox]

Currently the cleos plugin breaks when you pair it with any sort of resource provider plugin. It's because the cleos wallet doesn't support additional signatures being appended and send along with the push transaction command.

For this reason, it'd be good if the cleos wallet plugin could specify allowModify: false to prevent any plugins from modifying transactions that are being passed through to it.