Update webhook module for new clippy issues.

[kumarUjjawal]

Description
Rust 1.91.0 landed, Clippy introduced new lints (e.g. clippy::derivable_impls), so the pipeline started
failing for

meilisearch-rust/src/webhooks.rs

Lines 69 to 80 in e90a3c2

	#[derive(Debug, Clone, PartialEq, Eq)]
	enum HeadersUpdate {
	NotSet,
	Reset,
	Set(BTreeMap<String, Option<String>>),
	}
	impl Default for HeadersUpdate {
	fn default() -> Self {
	Self::NotSet
	}
	}

even though cargo clippy passed locally on Rust 1.89.0.

Expected behavior
Test-clippy should pass.

Current behavior
CI Fails

Screenshots or Logs

Environment (please complete the following information):

• OS: GitHub Actions ubuntu-latest
• Meilisearch version: N/A (not reached)
• meilisearch-rust version: v0.30.0