Connect to serial console of starting instance

[david-crespo]

In #2246 we added more deliberate handling of the serial console connection in certain states, and don't even bother trying to connect if it's not one of the states where we think we should be able to. However, we really want to be able to quasi-connect to a starting instance so you don't miss the beginning. If I misread the API code and it actually is possible to connect to a starting instance, we can just do that. But if that's not possible API-side, we can still work around that client-side by repeatedly attempting to connect (maybe with exponential backoff) while the instance is in the starting state.

console/app/api/util.ts

Lines 103 to 104 in 81bd2ab

	// https://github.com/oxidecomputer/omicron/blob/ebcc2acd/nexus/src/app/instance.rs#L1648-L1676
	serialConsole: ['running', 'rebooting', 'migrating', 'repairing'],

https://github.com/oxidecomputer/omicron/blob/b6857a11652c290b8731c2b81d8124f0594081d2/nexus/src/app/instance.rs#L1690-L1698