subscription callback support - update to latest version

[dariuszkuc]

While we are trying to move towards GA on the callback protocol (still in preview) we made few changes to the protocol.

Current changes*:

• router should specify maximum supported callback protocol version in all their messages
• subgraphs should specify their callback protocol version (in headers) in all their messages
• drop heartbeat message in favor of simpler check message (i.e. send heartbeats per each subscription)
  • heartbeat message required grouping subscription IDs per calling router without providing an easy way to group it (i.e. subgraphs would have to parse callback URLs to group heartbeats) which is error prone.**

See: apollographql/router#3884

*additional changes may be necessary as we finalize the protocol
**we may revisit this optimization in the future callback protocol versions

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