Fix/mqtt default subscription

[Umang01-hash]

Pull Request Template

Description:

• Add IsConnected() method to all pub-sub clients so that during subscription we can have a check of it before actually trying to subscribe.
• Add retryConnect to default MQTT client so that re-subscription happens after defaultClient reconnects.

Checklist:

• I have formatted my code using goimport and golangci-lint.
• All new code is covered by unit tests.
• This PR does not decrease the overall code coverage.
• I have reviewed the code comments and documentation for clarity.

Thank you for your contribution!

[vikash]

This looks unnecessary to be exposed. Subscribe can internally decide this. @Umang01-hash

[Umang01-hash]

This looks unnecessary to be exposed. Subscribe can internally decide this. @Umang01-hash

Sure @vikash will do it internally.

[coolwednesday]

If we are internally checking whether the client is connected or not...the IsConnected() function should be unexported.

[Umang01-hash]

If we are internally checking whether the client is connected or not...the IsConnected() function should be unexported.

Sure done.