Implement new apply method in ConfServiceInstanceProvider.Function

[ryanjbaxter]

No description provided.

[pantherdd]

What was the goal of this change? There is no description, no explanation of why this was necessary, while on the other hand it broke our integration with this library. ☹️

The regression this caused is that there were ways to customize the CuratorFramework instance that got used, e.g. by injecting a CuratorFrameworkCustomizer via a registry.registerIfAbsent call in a custom BootstrapRegistryInitializer that runs before ZookeeperConfigServerBootstrapper. Such customizations worked flawlessly until version 3.1.3, but in 3.1.4 they are now silently ignored. How are people supposed to do such things now? And what's the benefit of this change?

This change should be reverted; even if it makes sense to do this, it most definitely doesn't belong in a patch release.
Thanks.

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Update: opened issue #324 about this.

[pantherdd]

For the record (from #324 (comment)):

It was part of spring-cloud/spring-cloud-commons#1228 and spring-cloud/spring-cloud-config#1922