Add platform_ready support

[prspn]

This is a suggested change to OcaChannelManager, it adds a callback channel that signals platform_ready at the end of init, letting Flutter code synchronize stream subscriptions to when native listeners have been set up.

This PR is meant for main, but there is currently a similar kind of race in the client-connection-broker-2 branch; onControl will create a new OcaChannelManager but the Flutter side currently will not know when it is ready.

This PR will work for that case as well, at least until a better solution is available, the only change needed for that case would be to use a channelSuffix for the new platformStateChannel.

See what you think?

[lhoward]

Remind me as I've been out of this code for a while, this is moving the signaling from our UI into FlutterSwiftOCA? Is there a reason this is required or is this just a good refactoring that would be required (rather than optional) in the connection broker branch?

[prspn]

Yes, it would serve the same function as the current code does in the three runners (each is setting up a channel for this), so we could remove that. And it would also serve the same purpose for the connection broker branch, so it solves 4 problems in one location. It will co-exist with the current solution, though.