fix(poetry): skip if not installed with official script

[AThePeanut4]

What does this PR do

This skips the poetry self update step if poetry is not installed using the official installer script, preventing externally-managed-environment errors if it was installed through e.g. a package manager.

I've also added a config option (python.poetry_force_self_update) that bypasses the check to always run poetry self update - this is useful for manual installations.

Standards checklist

• The PR title is descriptive
• I have read CONTRIBUTING.md
• Optional: I have tested the code myself - tested on both Linux and Windows
• If this PR introduces new user-facing messages they are translated

Fixes #979

[SteveLauC]

Thanks!

[SteveLauC]

I took the liberty to document the new configuration entry in more details:)