Add extension options for specific workflow templates in the PTB

[ArBridgeman]

Summary

Projects using the PTB workflow templates often try to extend workflows.

In release 8.0.0, we added some extensions already:

• merge-gate-extension.yml
• fast-tests-extension.yml

It's anticipated that other projects might need a few more:

• cd.yml -> additional build objects that we do not want coupled with the existing ones

To Dos

• Perform a search to see what other extensions are needed
• Create a proposal and run it past @tkilias
• Create the additional workflow extensions and ensure proper test coverage
• Modify the documentation -> https://exasol.github.io/python-toolbox/main/user_guide/features/github_workflows/index.html#workflow-extensions