Change StepCustomization.content to be a list[StepContent] and improve docstrings and resolve highest security issue

[ArBridgeman]

Summary

It's a likely and already existing scenario that we may want to REPLACE or INSERT_AFTER multiple steps instead of just one (i.e. transformers-extension). As such, we want to modify StepCustomization.content to be a list[StepContent] and also make it clearer with docstrings what should be handled.

Since it's a small thing, let's also tackle our highest security issue:
https://sonarcloud.io/project/issues?impactSeverities=BLOCKER&issueStatuses=OPEN%2CCONFIRMED&types=VULNERABILITY&id=com.exasol%3Apython-toolbox&open=AZwJ_lFtAfuKZGDR2n3C