Feat: Added client destroy

[kwek20]

Description of change

client had an open handle, now you can close it!

Links to any relevant issues

fixes #520

Type of change

Choose a type of change, and delete any options that are not relevant.

• Bug fix (a non-breaking change which fixes an issue)

How the change has been tested

added it to test fixtures, no longer does jest complain! :carlos:

Make sure to provide instructions for the maintainer as well as any relevant configurations.

Change checklist

Tick the boxes that are relevant to your changes, and delete any items that are not.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes

[Thoralf-M]

In the tests it still prints Force exiting Jest: Have you considered using --detectOpenHandles to detect async operations that kept running after all tests finished?
Do we need the same for the SecretManager or does the destroy just need to be called in some other tests as well?

[kwek20]

Question: Setting to undefined is useful at all? I have no idea

[thibault-martinez]

Conflicts