Contracts: Return emitted events for dry-runs

[cmichi]

Contract dry-runs should return the events that would be emitted.

This will e.g. enable user interfaces to indicate better what the consequence of signing and submitting a particular transaction is.

A current UX movement for blockchains is to counter the "blind sign and submit" behavior ("yolo sign and submit"). We should provide better means to assess the effect of a transaction before submitting; returning the events that would be emitted would be a major step towards that.

[kianenigma]

Would be good if the system_dryRun does the same.

[xlc]

Yeah we should just improve system_dryRun paritytech/polkadot-sdk#341

[athei]

Please note we can't just use system_dryRun for dry running contracts. We need to return dispatchable specific data from the dry run. As long as dispatchables can't return data we need out custom dry run runtime API.