[3007.x] Add started event to PublishServer

[dwoz]

• Add a started event that gets set after the server transport is ready for clients to connect.
• Wait on publish servers start while the master process is starting up.

What does this PR do?

What issues does this PR fix or reference?

Fixes #66993

Previous Behavior

Publish clients in the master process could encounter exceptions while trying to connect.

New Behavior

Publish clients within the master processes connect without error.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

[dwoz]

1. I'm not sure the rational for passing this as an argument. What's the use case for using an arbitrary object? I think it would be better to have a separate method that you can give a timeout to so that transports can implement their own methodology and better integrate with [3007.x] transports: fix and normalize PublishServer method signatures #66751. Something like def wait_startup(self, timeout): bool
2. I think [3007.x] transport: tcp check that pull_path exists before chmod #66931 is needed for the tests to fully pass.

I agree a method like wait_startup might make sense in the future.