Update visibility for IdentityProvider property inside Registration class for Saml2 relying party properties

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Comment From: wilkinsona

Thanks for the PR. As noted in the documentation, @ConfigurationProperties classes available in Spring Boot are for internal use only. The properties that map to the class, which are configured via properties files, YAML files, environment variables etc., are public API but the content of the class itself is not meant to be used directly.

Given that, I don't think we necessarily want to make the method public if it does not need to be. There's a mixture of public and package-private methods at the moment. Let's use #19194 to address that. Thanks anyway.

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