Add KV key codec support for KeyValueStore Extensions

[mtmk]

Adds transparent key encoding/decoding for NATS KV stores, ported from orbit.go's kvcodec package.

Features:

• INatsKeyCodec / INatsFilterableKeyCodec interfaces for custom codecs
• NatsBase64KeyCodec - URL-safe Base64 encoding (handles special characters in keys)
• NatsPathKeyCodec - translates /path/style keys to NATS .subject.style
• NatsKeyChainCodec - compose multiple codecs
• NatsKVCodecStore - wraps INatsKVStore with codec support
• Extension methods: WithBase64Keys(), WithPathKeys(), WithKeyCodec()

Usage:

using Synadia.Orbit.KeyValueStore.Extensions.Codecs;

// Base64 encoding for keys with special characters
var store = rawStore.WithBase64Keys();

// Path-style keys: "/users/123" → "_root_.users.123"
var store = rawStore.WithPathKeys();