Store events, add a read_event method.

[afck]

Motivation

Event streams will soon replace broadcast channels, and also enable other blockchains to more easily interpret Linera data, e.g. to facilitate bridges.

Proposal

Introduce a new type:

struct EventId { 
  chain_id: ChainId,
  stream_id: StreamId, // (<applicationId>, <streamName>) tuple
  key: Vec<u8> // index of the event in the particular stream
}

Add Event(EventId, Vec<u8>) variant to OracleResponse.

Collect events from the executed block and store them in storage.

Add a read_event(event_id) method to the contract and service runtimes.

Test Plan

TBD

Release Plan

• Need to bump the major/minor version number in the next release of the crates.
• Need to update the developer manual.

Links

• Closes
• • Add events to storage separately. #2305,
• • Add a read_event function to the contract runtime. #2308.
• reviewer checklist