feat: add XML Codec module (#754)

[Abu1982]

XML Codec for ZIO Schema

Closes #754

Summary

This PR adds a new zio-schema-xml module that provides XML encoding and decoding support for ZIO Schema, following the same patterns as existing codecs (JSON, MessagePack, Protobuf, etc.).

Implementation

Module structure:

• XmlCodec.scala - Entry point providing BinaryCodec[A] instances via implicit xmlCodec[A]
• XmlEncoder.scala - Encodes ZIO Schema values to XML strings with proper escaping
• XmlDecoder.scala - Decodes XML strings back to ZIO Schema values using scala-xml
• XmlCodecSpec.scala - Comprehensive roundtrip tests

Supported types:

• All primitive types (String, Int, Long, Boolean, Double, Float, BigDecimal, BigInteger, UUID, Char, Binary/Base64)
• All temporal types (LocalDate, LocalTime, LocalDateTime, Instant, Duration, ZonedDateTime, OffsetDateTime, OffsetTime, Period, Year, YearMonth, MonthDay, DayOfWeek, Month, ZoneId, ZoneOffset)
• Currency type
• Case classes (CaseClass0 through CaseClass22 via generic Record API)
• Sealed traits / enums (Enum1 through EnumN)
• Collections (List, Set, Map, Sequence)
• Optional values
• Either and Fallback
• Tuples
• Nested/recursive types via Schema.Lazy
• GenericRecord
• DynamicValue
• Transform schemas

XML format:

• Records encode fields as child elements: <root><name>Alice</name><age>30</age></root>
• Sequences use <item> elements: <root><item>1</item><item>2</item></root>
• Maps use <entry><key>k</key><value>v</value></entry> structure
• Enums use a type attribute: <root type="Red"><value/></root>
• Optional None renders as self-closing: <field/>
• XML declaration included: <?xml version="1.0" encoding="UTF-8"?>
• Special characters properly escaped (&, <, >, ", ')

Dependencies:

• org.scala-lang.modules:scala-xml:2.3.0 (JVM only, like MsgPack/Thrift/Avro/Bson modules)

Testing

Roundtrip tests covering primitives, case classes, nested structures, collections, optionals, either, enums, tuples, temporal types, and XML special character handling.

[hriszc]

I reviewed the current XML codec implementation and found two round-trip edge cases that look merge-blocking:

• empty optional payloads like Some("") and Some(Nil) can collapse to None
• string decoding trims surrounding whitespace, so some values do not round-trip exactly

I prepared a focused follow-up patch here:

• patch gist: https://gist.github.com/hriszc/6289f50e6f8e9a49ec42c36af109041b

What it changes:

• encodes Option values explicitly with a <value> wrapper and nil="true" for None
• keeps a compatibility path for previously encoded optional payloads
• preserves leading/trailing whitespace for String and Char
• adds regression tests for Some(""), Some(Nil), and strings with surrounding whitespace
• wires zioSchemaXml into the standard JVM test alias/docs aggregation

I did not run the JVM suite locally because the environment I used did not have a usable Java runtime, so this should still be validated in CI/local JVM once applied.

[hriszc]

Follow-up on the XML codec thread: the patch comment I posted earlier is still the intended delta from my side, focused on Option encoding/decoding stability, whitespace preservation, and wiring the XML module into the standard test path.\n\nThe PR itself is green. If maintainers are open to folding those fixes into this branch, I can keep iterating from that direction rather than opening a duplicate PR.