Add AnyUtf8: one logical type for any UTF-8 encoding

[emilk]

The string sibling of AnyBinary / AnyList: a quiver-only logical type that accepts any of arrow's UTF-8 encodings and reads them uniformly as &str.

Logical type	Accepts	Element value
AnyUtf8	Utf8, LargeUtf8, Utf8View	&str

let column = Column::<AnyUtf8>::try_from(array)?; // Utf8 / LargeUtf8 / Utf8View
assert_eq!(column.value(0), "alice");

Details

• Enum AnyTypedUtf8 over the three arrow string arrays; downcast dispatches on array.data_type() and reads every element as &str.
• Parse-only: implements LogicalType and RefType (so try_from/reading/column[i] work) but not ConcreteType — no single arrow datatype, so no from_values/Default/schema. Build a concrete encoding such as Column<Utf8>.
• (No fixed-size variant exists for UTF-8 in arrow, so — unlike AnyBinary — there's nothing extra to include.)
• Reads stay O(1)/zero-copy; nullability via Option<AnyUtf8> like any other column.

Verification

• cargo clippy --all-features --all-targets clean, cargo fmt --all applied
• cargo test --all-features green, incl. a new any_utf8_columns test: all three encodings read uniformly, RefType indexing, non-string rejected, null handling (Option<AnyUtf8> + non-nullable rejection)
• cargo doc --document-private-items -D warnings clean
• README: added to the supported-types table and the AnyList section

🤖 Generated with Claude Code