Add support for disabling stack trace collection

[pvital]

Description

This PR adds support to disable the stack trace collection in the Python sensor.

Changes

1. Added "stack" to the SPAN_CATEGORIES list in src/instana/util/config.py
2. Modified the _add_stack method in src/instana/tracer.py to check if stack trace collection is disabled
3. Added tests for the "stack" category in tests/test_span_disabling.py
4. Added a test to verify that stack trace collection can be disabled in tests/test_tracer.py

How to Test

1. Set the environment variable INSTANA_TRACING_DISABLE=stack
2. Run a Python application with the Instana sensor
3. Verify that no stack traces are collected in the spans

[github-actions]

❌ @pvital the signed-off-by was not found in the following 1 commits:

• 5052850: Add support for disabling stack trace collection

📝 What should I do to fix it?

All proposed commits should include a sign-off in their messages, ideally at the end.

❔ Why it is required

The Developer Certificate of Origin (DCO) is a lightweight way for contributors to certify that they wrote or otherwise have the right to submit the code they are contributing to the project. Here is the full text of the DCO, reformatted for readability:

By making a contribution to this project, I certify that:

a. The contribution was created in whole or in part by me and I have the right to submit it under the open source license indicated in the file; or

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c. The contribution was provided directly to me by some other person who certified (a), (b) or (c) and I have not modified it.

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Contributors sign-off that they adhere to these requirements by adding a Signed-off-by line to commit messages.

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Git even has a -s command line option to append this automatically to your commit message:

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