Enable PowerShell Core support in Kubernetes Executor

Description

Enable PowerShell Core support in the GitLab Runner Kubernetes Executor.

Proposal

• Have kubernetes executor choose between pwsh and bash depending on the user config.toml. We need to remove any assumptions we have that we are using a bash shell.
• Choose the correct helper image, the helper image should already be done in #4021 (closed)

Links to related issues and merge requests / references

• Investigation issue #3291 (closed)
• PoC MR !1855 (closed)