Chore: enable serde feature in example code

[getong]

Checklist

• Updated guide with pertinent info (may not always apply).
• Squash down commits to one or two logical commits which clearly describe the work you've done.
• Unittest is a friend:)

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This change is 

[SteveLauC]

Hi, I am not quite sure about why we didn't see these errors in the past, but this PR should not be considered a fix for them, we should not add a feature flag that won't be used but simply for a workaround of a clippy lint, especially in example code. If this derive does not work at all, maybe we should remove it🤔

[getong]

Hi, I am not quite sure about why we didn't see these errors in the past, but this PR should not be considered a fix for them, we should not add a feature flag that won't be used but simply for a workaround of a clippy lint, especially in example code. If this derive does not work at all, maybe we should remove it🤔

need discussion

[drmingdrmer]

The #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))] is a misuse and should be removed.
I found this issue too and have had it fixed in: #1268 :

https://github.com/databendlabs/openraft/pull/1268/files#diff-8f8d2d841365befc4ced06a6646431e67d0d5a0632a7f96573ce97fb69a47ad5