feat(callback): support configurable heartbeat interval

[dariuszkuc]

Update protocol to support configurable heartbeat interval. In order to keep backwards compatibility with older version of protocol, if no heartbeat value is provided, we default to 5s.

Updated protocol to also support both camelCase and snake_case extension values.

[github-actions]

Apollo Federation Subgraph Compatibility Results

Federation 1 Support	Federation 2 Support
_service 🟢 @key (single) 🟢 @key (multi) 🟢 @key (composite) 🟢 repeatable @key 🟢 @requires 🟢 @provides 🟢 federated tracing 🟢	@link 🟢 @shareable 🟢 @tag 🟢 @override 🟢 @inaccessible 🟢 @composeDirective 🟢 @interfaceObject 🟢

Learn more:

• Apollo Federation Subgraph Specification
• Compatibility Tests

[bnjjj]

I think that would be interesting to handle the case when heartbeat_interval_ms is not provided. To have a smooth transition for people using an older version of the router and stay backward compatible during few weeks. What do you think @dariuszkuc ?