Add ability to handle external transaction intents

[sohkai]

It would be interesting to let a frontend create transactions for external contracts (and not just the app's listed contract) in cases where an app needs to communicate to multiple contracts (e.g. in patterns where the initial app contract is a factory for a few other contracts).

The built transaction should probably still go through the transactions observable, and any frontends should display them with slightly different information (e.g. "This will send a transaction to an external contract at 0x...")

[sohkai]

From #43:

In the case of sending transactions, it should be handled differently than current 'intents' and allow to perform the action directly without calculating any fwd path. The signer in the wrapper should let the user know it is not a 'normal action' being performed and it could have consequences. In the case of known actions such as token transfers, we would be able to provide a description for it.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for contributing to Aragon! 🦅