Consolidate std::time::Duration parsing

[jmacd]

We have at least three mechanisms in our otap-dataflow crates for parsing a Duration in a human-readable form.

1. #[derive(JsonSchema)] on the config struct
2. #[serde_as(as = "DurationSecondsWithFrac<f64, Flexible>")] on the field
3. #[serde(with = "humantime_serde", default = "default_timeout_duration")] on the field

This is unintentional! This is silly.
Let's fix it.