feature: built-in "heartbeat" support

[joshmossas]

Describe the feature

Arri-RPC clients should have built-in heartbeat detection for all realtime features (SSE and Websockets).
Arri-RPC servers should have a way of indicating how often it will send a "heartbeat" to the client(s) and they should automatically send heartbeat events according to that specified timeframe.

Initial Proposal:

The server will send a "heartbeat-interval" header. The value of the header will be an integer indicating the number of milliseconds between each heartbeat message.

When the "heartbeat-interval" is present the generated arri clients will set up a timer that will reopen a connection with the server if no message is received before the end of the timer. Whenever a new message is receive the timer is reset. When the connection is closed by the client or the server the timer is cancelled.

Additional context

Server Progress:
Servers that implemented the "heartbeat-interval" response header and automatically send a "ping" message to keep the connection alive

lang	completed
ts	yes
go	yes

Client Progress:

Clients that watch for the "heartbeat-interval" response header and automatically reconnect if they haven't received a message within the specified timeframe.

lang	completed
ts	yes
dart	yes
rust
kotlin
swift

Would you be willing to implement this feature?
Yes

[joshmossas]

#174 adds server support for heartbeats + client support in the Dart and TS clients