[C++] Add isConnected() method

[BewareMyPower]

Motivation

Add isConnected() method to Producer, Consumer and Reader in C++ client.

Modifications

• Implement isConnected() method that checks the network connection and state and add related tests.
• Unify the signature of overrided methods to avoid compile warnings like overrides a member function but is not marked 'override' [-Winconsistent-missing-override]**.

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• ProducerTest.testIsConnected
• ConsumerTest.testIsConnected
• ReaderTest.testIsConnected

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API: (yes)
• The schema: (yes / no / don't know)
• The default values of configurations: (yes / no)
• The wire protocol: (yes / no)
• The rest endpoints: (yes / no)
• The admin cli options: (yes / no)
• Anything that affects deployment: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes)
• If yes, how is the feature documented? (docs)

[Anonymitaet]

@BewareMyPower docs should be updated as well?

[BewareMyPower]

@BewareMyPower docs should be updated as well?

I've already added it in C++ headers.