Switch /plugins to the derived endpoint & request builder implementation

[oguzkocer]

Removes the manual endpoint & request builder implementation for /plugins in favor of the derived implementation from #[derive(WpDerivedRequest)].

Updates WpDerivedRequest to generate the functions to accept &Foo instead Foo in the given "/foo/<bar>" url input.

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To Test

• make test-server && make dump-mysql && make backup-wp-content-plugins
• cargo test --test '*' -- --test-threads 1

[jkmassel]

Reviewed in sync call – this is a very simple change that mostly just removes hand-copied code