Add optional list() method to KvStore interface

Add an optional list() method to the KvStore interface for enumerating
entries by key prefix.  This enables efficient prefix scanning which is
useful for implementing features like distributed trace storage, cache
invalidation by prefix, and listing related entries.

- Add KvStoreListOptions interface
- Add KvStoreListEntry interface
- Add list() method to KvStore interface
- Implement list() in MemoryKvStore

Resolves #498

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>