Consider issuing warning if using spring reserved configuration keys

[philwebb]

Keys in the spring. namespace (and perhaps server.) are being used by other projects. It's possible that this will cause problems if in future releases we decide to add extra keys that the project already uses.

At a minimum we should document best practices, but it might also be possible to issue a warning if a reserved prefix is used by a @ConfigurationProperties class outside of a org.springframework package.

If we do add such a feature, we should also offer an opt-out.