Expand the post message API to support automatically expanding track groups

[kenzieschmoll]

Some notes from an offline discussion between @stevegolton and myself.

We can add a field 'expandAllTrackGroups' to the post trace interface, and when true, loop through all groups and call this: https://github.com/google/perfetto/blob/main/ui/src/common/actions.ts#L396. Just like here: https://github.com/google/perfetto/blob/main/ui/src/common/plugins.ts#L265

There's also a command to expand all groups which you can call programmatically using: globals.commandManager.runCommand('dev.perfetto.CoreCommands#ExpandAllGroups'), but we might run into races between plugins registering commands and postMessage handler getting run.