Using closeable in opamp client api

Use closeable in both `OpampClient` and `RequestService` and remove their `start` and `stop` methods, so that the implementations start their processes during creation.

Based on these comments:

- https://github.com/open-telemetry/opentelemetry-java-contrib/pull/1835#discussion_r2097120696
- https://github.com/open-telemetry/opentelemetry-java-contrib/pull/1928#discussion_r2123380213
            