Add JSpecify nullability annotations to the codebase.

Loose ends:

• Add dependency management for JSpecify #46776
• Rely on org.springframework.core.Nullness to determine if an Actuator endpoint is optional #46593
• Remove nullAwayVersion and errorProneVersion gradle properties after Upgrade to ErrorProne 2.41.0 spring-gradle-plugins/nullability-plugin#5 and Upgrade to NullAway 0.12.8 spring-gradle-plugins/nullability-plugin#4 is resolved and we're using 0.0.3 of the plugin
• Add ArchUnit / Checkstyle rule to verify that all packages are @NullMarked
• Review org.springframework.boot.jpa.EntityManagerFactoryBuilder.Builder#setPackagesToScan