Add selective preload of script modules for Bash and PowerShell

[HuyPhanNguyen]

Summary

• Adds Octopus.Action.Script.PreloadScriptModules variable to control which script modules are auto-imported before a user script runs
• Bash: selected modules are sourced in the bootstrapper before the user script (previously no auto-sourcing)
• PowerShell: when variable is set, only listed modules are auto-imported. When unset, existing behavior preserved (all modules imported)
• Variable supports comma/semicolon delimited lists and can be scoped per step/environment like any Octopus variable

Builds on the concept from #1730 (Bash-only) and extends it to PowerShell with backwards-compatible selective import.

Test plan

• Bash: single module preload works
• Bash: multiple modules preload in order
• Bash: delimiter edge cases (comma, semicolon, spaces, empty entries, missing modules)
• PowerShell: selected module imports and functions are available
• PowerShell: unselected modules are not auto-imported
• PowerShell: no variable set = all modules still auto-imported (existing behavior)