feat: add is_opened flag to socket base class

[domire8]

Description

• Notify if open() has not been called #65

This PR solves the issue by applying to proposed change in the parent topic to the base class, with one difference: the receive/send function throw an exception if the socket hasn't been opened. Otherwise it might be really hard to understand if it failed because there is no connection or because it was not opened.

Quite a big PR because this required changes in all derived socket classes and I updated the tests quite a bit.

Review guidelines

Estimated Time of Review: 15 minutes
Review by commit possible

Checklist before merging:

• Confirm that the relevant changelog(s) are up-to-date in case of any user-facing changes

[JasperTan97]

Builds fine for me!

[eeberhard]

Great work! An elegant solution. Thanks for also updating the tests