samples: rpc: log during method invocations

[mniestroj]

Right now this sample was silent on the Zephyr console, so the only way to tell whether RPC was executed was using REST API and/or web console. This however does not tell whether tested board was the one responding with correct results.

Add debug logs, so that every RPC invocation results in clear notification on Zephyr console.

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit c2cbe4f):

https://golioth-zephyr-sdk-doxygen-dev--pr356-samples-rpc-log-oub3nt68.web.app

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