feat: add optional serde Serialize/Deserialize feature (#115)

[MukundaKatta]

Summary

Closes #115. Adds an opt-in serde feature that derives Serialize/Deserialize on Row, Column, ColumnType, TokenRow, ColumnData, Numeric, time types, and XmlData/XmlSchema — so query results can be shipped over the network as JSON. Off by default.

Types covered (all gated behind feature = "serde")

• Row, Column, ColumnType (src/row.rs)
• TokenRow<'a> (src/tds/codec/token/token_row.rs)
• ColumnData<'a> (src/tds/codec/column_data.rs) — Cow<str>, Cow<[u8]>, Uuid, etc.
• Numeric (src/tds/numeric.rs)
• DateTime, SmallDateTime, Date, Time, DateTime2, DateTimeOffset (src/tds/time.rs)
• XmlData, XmlSchema (src/tds/xml.rs)

Feature gating

serde = ["dep:serde", "uuid/serde"] with serde requiring ["derive", "rc"] (rc for Arc<XmlSchema>). Off by default; added to all group. All types use #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))] — derive was sufficient throughout, no custom impls needed.

Test plan

• tests/serde.rs — 17 round-trip tests through serde_json for ColumnType, Column, ColumnData (int/null/string/binary/guid/bool/float/numeric/datetime), Numeric, DateTime, all tds73 time types, XmlData, TokenRow, and a Row-shape flow
• cargo build (default), cargo build --features serde, cargo build --features all all succeed
• Lib tests: 117 pass with and without serde
• cargo fmt --all clean

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