Add NvramStore for router NVRAM-backed storage

[rveznaver]

Summary

Adds NvramStore, a Store implementation that uses router NVRAM for persistent storage. This is useful for running HomeKit accessories on routers with limited flash storage where filesystem-based storage isn't ideal.

Key features:

• Configurable key prefix to support multiple independent accessories on the same router
• Minimises flash writes by only committing to flash when pairing data changes
• Hex-encodes binary data (configHash) for NVRAM compatibility
• Converts pairing keys to readable UUID format to stay under NVRAM's 64-char key limit

Commit strategy

To preserve flash longevity, non-critical data (uuid, keypair, schema, version, configHash) is written to NVRAM RAM but not committed to flash. Only pairing changes trigger a flash commit. This means:

• Normal startup: 0 flash writes
• Per pairing added/removed: 1 flash write

If power is lost before first pairing, non-pairing data is regenerated on next startup. Once paired, the commit includes all pending changes, keeping keypair and pairing in sync.

Usage

store := hap.NewNvramStore("hap_")
server, err := hap.NewServer(store, accessory.A)