fix: return topic subscribe error immediately if subscription limit reached

[anitarua]

addresses #55

Checks that the first message is a heartbeat before returning a TopicSubscription or TopicSubscribeError response. Now, if a user tries to subscribe >100 times, they will get a TopicSubscribeError saying they've reached the 100 subscriber limit.

Also did some testing and determined that the keepalive settings seemed to be interfering with the ability to reconnect. Keepalive settings might not play well with the default backoff and retry strategy that dart-grpc uses, so removed for now.

Before (with keepalive settings):

FINE: 2024-01-04 15:20:46.537611: topic client received a heartbeat
FINE: 2024-01-04 15:20:53.061621: Attempting to reconnect after receiving error: gRPC Error (code: 2, codeName: UNKNOWN, message: HTTP/2 error: Connection error: Connection is being forcefully terminated. (errorCode: 10), details: null, rawResponse: null, trailers: {})
FINE: 2024-01-04 15:20:53.065014: Error reconnecting: Error connecting: Connection shutting down

After (without keepalive settings):

FINE: 2024-01-05 10:00:06.773596: topic client received a heartbeat
FINE: 2024-01-05 10:06:10.777344: Attempting to reconnect after receiving error: gRPC Error (code: 2, codeName: UNKNOWN, message: HTTP/2 error: Connection error: Connection is being forcefully terminated. (errorCode: 10), details: null, rawResponse: null, trailers: {})