fix: use pub(crate) visibility for RecordingTimestampGenerator

The struct and its methods only need crate-level visibility since
the module itself is pub(crate). Using pub triggers the
unnameable_types lint.

Author: dkropachev

scylla-rust-wrapper/src/timestamp_generator.rs

@@ -17,6 +17,7 @@ pub enum CassTimestampGen {
 /// A wrapper around `MonotonicTimestampGenerator` that records all generated timestamps.
 /// This is used for integration testing purposes only.
 #[cfg(cpp_integration_testing)]
+#[allow(unnameable_types)]
 pub struct RecordingTimestampGenerator {
     inner: MonotonicTimestampGenerator,
     timestamps: std::sync::Mutex<Vec<i64>>,