Join all connections at the given identity (or just one if endpointId provided) to channel contextGroupId. If no target is specified, it adds the sender to the channel.

This is a wrapper for interop.joinContextGroup.

Example

await fdc3.joinChannel('red');
await fdc3.addContextListener('fdc3.instrument', console.log);
await fdc3.broadcast({type: 'fdc3.instrument', id: { ticker: 'AAPL' }});