Declarative configuration: move `.instrumentation.java.agent` node to `.distribution.java_agent`

or `.distribution.javaagent`

See https://github.com/open-telemetry/opentelemetry-specification/pull/4800#discussion_r2632786351 for how to access `.distribution` node.

