RUST-1748 Convert unsigned serde helpers to use serde_conv

[JamieTsai1024]

Converted unsigned (u32, u64) converter functions in src/serde_helpers.rs to use serde_conv macro for serializing and deserializing optional and vector fields with serde_with.

• Added tests for <T>, Option<T>, and Vec<T> fields (de)serialized using structs
• Added documentation for modules and structs

Old	New
u32_as_timestamp	u32::AsTimestamp (u32 → Timestamp)
timestamp_as_u32	u32::FromTimestamp (Timestamp → u32)
u32_as_f64	u32::AsF64 (u32 → f64)
serialize_u32_as_i32	u32::AsI32 (u32 → i32)
serialize_u32_as_i64	u32::AsI64 (u32 → i64)
u64_as_f64	u64::AsF64 (u64 → f64)
serialize_u64_as_i32	u64::AsI32 (u64 → i32)
serialize_u64_as_i64	u64::AsI64 (u64 → i64)

Previously:

• Rewrote serde converters as structs with SerializeAs and DeserializeAs traits to support Optional and Vector conversions in PR #559
• Adapted serde_conv macro to support Rustdocs, refactored all ObjectId converters in PR #566
• Refactored all DateTime converters PR #570

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