Support TURN REST API (Shared Secret) Authentication

[lowlyocean]

Closes #500

Pion's turn/v4 library (already an indirect dependency) provides GenerateLongTermTURNRESTCredentials() which generates time-limited HMAC-SHA1 credentials from a shared secret — exactly what coturn expects.

3 files with minimal changes:

webrtc.go : custom ICEServerConfig struct with shared_secret and credential_ttl fields, a resolveICEServers() function that generates fresh credentials per connection, and moving config resolution into the PeerConnection closure
go.mod : promote pion/turn/v4 to direct dependency
schema.json : add the new fields to the JSON schema

Usage:

webrtc:
  ice_servers:
    - urls: [stun:stun.l.google.com:19302]
    - urls: [turn:my-coturn:3478]
      shared_secret: "my-secret"