feat: add Spring ConfigProvider example

[zeitlinger]

Summary

• Add a ReadInstrumentationConfig Spring component that demonstrates reading an instrumentation setting through the ConfigProvider bean.
• Update the Spring declarative configuration example to the OpenTelemetry Spring Boot Starter 2.30.0 BOM, which provides the ConfigProvider bean.

Test plan

• mise run lint ✅
• ./gradlew assemble passed before the required 2.30.0 dependency bump with the same source change.
• Final Gradle validation is pending Maven Central publication of the newly released 2.30.0 BOM; it currently returns 404.