RUST-1748 Apply formatting changes to ObjectId

[JamieTsai1024]

• Add rustdoc comments to object_id module and contained structs AsHexString and FromHexString
• Move tests to match ordering of modules in src/serde_helpers.rs
• Shorten tests for *_optional_some, *_vector, and deserializing
• Move location of #[cfg(feature = "serde_with-3")] in tests to reduce indentation

Condense oid_optional_some:

    // Before 
    assert_eq!(
        doc.get("oid_optional_some"),
        Some(&Bson::ObjectId(oid)),
        "Expected serialized oid_optional_some to match original."
    );

    // After 
    match doc.get("oid_optional_some") {
        Some(Bson::String(value)) => {
            assert_eq!(
                *value,
                oid.to_hex(),
                "Expected serialized oid_optional_some to match original ObjectId."
            )
        }
        _ => {
            panic!("Expected serialized oid_optional_some to be a BSON String.")
        }
    }

Condense *_vector:

    // Before 
    let expected_oid_vector: Vec<Bson> = b
        .oid_vector
        .into_iter()
        .map(|oid| Bson::String(oid.to_hex()))
        .collect();

    // After
    let expected_oid_vector: Vec<Bson> = vec![Bson::ObjectId(oid)];

Condense deserialization:

    // Before 
    // Deserialize the BSON back to the struct
    let b_deserialized: B = deserialize_from_document(doc).unwrap();

    // Validate deserialized data
    assert_eq!(
        b_deserialized.oid, oid,
        "Expected deserialized oid to match the original."
    );

    assert_eq!(
        b_deserialized.oid_optional_none, None,
        "Expected deserialized oid_optional_none to be None."
    );

    assert_eq!(
        b_deserialized.oid_optional_some,
        Some(oid),
        "Expected deserialized oid_optional_some to match the original."
    );

    assert_eq!(
        b_deserialized.oid_vector,
        vec![oid],
        "Expected deserialized oid_vector to match the original.."
    );

    // After 
    // Validate deserialized data
    let a_deserialized: A = deserialize_from_document(doc).unwrap();
    assert_eq!(
        a_deserialized, a,
        "Deserialized struct does not match original."
    );

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 in PR #570
• Converted all unsigned (u32 and u64) helpers in PR #575