Add go-idiomatic ESP-NOW wrappers

[sparques]

This builds on the previous ESP-NOW work:

• New managed ESP-NOW API: ESPNow, Peer, ESPNowAddr, peer config, broadcast support, close/deadline handling, and net.PacketConn implementation.
• New PeerStream adapter that turns a packet peer into an io.ReadWriter, including write fragmentation.
• ESP-NOW callbacks now fan out to both existing user handlers and the managed ESPNow router.
• Expanded package docs with ESP-NOW managed API examples.

[deadprogram]

@sparques can you please rebase this branch against the latest main now that your other changes have been merged? Thanks.

[sparques]

@deadprogram Updated!

[deadprogram]

Why not add this as an example in examples/espnow? That should also build that example as part of the smoke tests. Also makes it a lot easier to try it and use it for things.

[sparques]

Example added.

[deadprogram]

Thank you very much for this addition @sparques now squash/merging.