fix: Fix multiple parallelization issues

[PeterSchafer]

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What does this PR do?

This PR addresses multiple issues related to parallel execution of the CLI as a standalone executable on Windows OS as well as through the language-server command on any OS. The following changes are combined since they are strongly coupled.

• Replace singleton usage for the internal proxy by a singleton for the internal certificate authority, which showed to be the actual shared instance that required this.
• Replace implicit cleanup of temporary directories by an explicit removal, which can be triggered through the workflow engine.

Risk:

• For CLI standalone execution, the risk should be low as the main change here is less cleanup.
• For the language-server command, the risk is that in some edge cases, like crashes, temporary files are not cleaned up. Which is in a lot of cases acceptable.

Where should the reviewer start?

How should this be manually tested?

Any background context you want to provide?

What are the relevant tickets?

Screenshots

Additional questions

[cmars]

I have concerns with the acceptance tests -- we won't get a clear enough signal out of them to test race conditions properly.

I'd prefer to improve the Go tests to the point that we're confident in the primitives we've built upon.