Add DeriveConfig support for Map with non-String keys via ConfigKeyDecoder

[loathwine]

Summary

Adds ConfigKeyDecoder[K] typeclass to support Map[K, V] where K is not String, fixing #1381.

Previously, DeriveConfig only supported Map[String, V] via Config.table. This adds a ConfigKeyDecoder trait that decodes map keys from their string representation, with built-in instances for String, UUID, Int, and Long.

This is particularly useful for libraries like neotype where wrapper types are commonly used as map keys — see kitlangton/neotype#413 for a concrete use case.

Usage

// Built-in key types work out of the box
case class MyConfig(entries: Map[UUID, String])
val config = deriveConfig[MyConfig]

// Custom key types need a ConfigKeyDecoder instance
case class EntryId(value: String)
given ConfigKeyDecoder[EntryId] = (key: String) => Right(EntryId(key))

Changes

• New ConfigKeyDecoder trait with built-in instances for String, UUID, Int, Long
• Replaced Map[String, V]-only mapDesc with generalized Map[K, V] version using ConfigKeyDecoder[K]
• Backwards compatible — Map[String, V] continues to work as before
• Supports Scala 2.12, 2.13, and 3

Test plan

• Map[String, Int] — backwards compatibility
• Map[UUID, String] — UUID keys parsed from config
• Map[Int, String] — Int keys parsed from config
• Invalid key — error propagation
• Case class containing Map[UUID, CaseClass] — automatic derivation

[CLAassistant]

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