feat: add flat string serde support for Date, Time and DateTime

[ZialeHub]

Summary

Currently, serializing a Date, Time, or DateTime produces a nested JSON object:

{ "date": "2023-10-09" }
{ "time": "12:21:46" }
{ "datetime": "2023-10-09T00:00:00" }

The vast majority of REST APIs, databases, and configuration formats represent temporal values as flat strings:

"2023-10-09"
"2024-10-31 06:32:28"

Without flat serde support, consumers must wrap timeflow types in newtype structs or write fully custom Deserialize impls, which directly undermines the library's goal of reducing boilerplate.

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Proposed API

serde(with) modules (recommended)

Expose dedicated modules usable with #[serde(with = "...")]:

use timeflow::prelude::*;

#[derive(Serialize, Deserialize)]
struct Event {
    #[serde(with = "timeflow::serde::flat")]
    begin_at: Date,

    #[serde(with = "timeflow::serde::flat")]
    ends_at: DateTime,
}

Serializes to:

{ "begin_at": "2024-10-31", "ends_at": "2024-10-31 06:32:28" }

A companion flat_opt module should handle Option<Date> / Option<DateTime> fields, serializing None as JSON null.

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Implementation Notes

• The flat deserializer must use the active BASE_*_FORMAT globals so it respects any SpanBuilder configuration set at startup.
• Both serialize and deserialize functions must be provided inside the module so #[serde(with = "...")] works without splitting into serialize_with / deserialize_with.
• The existing default (nested-object) serde behaviour must remain unchanged for full backwards compatibility.
• The existing deserialize_with_format / serialize_with_format custom-format hooks should remain unaffected.

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Acceptance Criteria

• #[serde(with = "timeflow::serde::flat")] compiles and works on Date, Time, and DateTime fields
• Flat serialization produces a bare JSON string, not a nested object
• Flat deserialization parses a bare JSON string using the active BASE_*_FORMAT
• timeflow::serde::flat_opt handles Option<T> fields correctly (None → null, Some(t) → "string")
• Existing default (nested) serde behaviour is unchanged — no breaking change
• Unit tests cover: serialize, deserialize, round-trip, invalid input, and Option variants
• README and module-level docs updated with a usage example