Remove isImmutable and getPrefix from OriginLookup

[philwebb]

The OriginLookup interface has isImmutable and getPrefix methods that were added to fix a package tangle. They don't really belong in this interface and it would be good to find a better way to solve the problem in 4.0

[philwebb]

We might also want to look at the fix for #45387

[philwebb]

Changing EnvironmentPostProcessor might be the best way to do this. Perhaps supporting SpringApplication as a constructor argument or moving the interface somewhere else.