Add custom Duration and Timestamp types for conversion with serde

[fore5fire]

Currently chrono::Duration doesn't implement Seralize and chrono::DateTime serializes to an ISO 8601 string. This change is inspired by the toml crate's approach to preserving date formats across functions available in the Serialize trait to ensure that durations and timestamp are converted into the appropriate CEL types.

Using the newly exposed Duration and Timestamp types is only necessary for types using serde to convert to CEL, but maybe it could be confusing to keep track of where to use the chrono types vs the cel_interpreter types? But it seems like an unnecessary breaking change to use the new types in Value::Duration and Value::Timestamp 🤷🏼

[fore5fire]

I did some research and found the clever idea to use serde's newtype_struct name field as the indicator to do special type handling. Since serde's official recommendation is to silently unwrap newtypes, it's a great way to signal to a custom Serializer but remain compatible with most other implementations

[fore5fire]

Updated for the new chrono feature