SpringBoot Replace POJO used as nested namespace in spring-boot by an immutable contract

The spring-boot module defines a Ssl and Compression object that looks like something that's part of a @ConfigurationProperties.

It does but it's also used by the lower-level API. We should probably stop doing that and replace that by an interface in spring-boot and a dedicated implementation in spring-boot-autoconfigure. This will also reduces the metadata problem exposed in #14669 as the annotation processor will be able to detect the description.