add config section feature_flags

[BenjaminBoehm]

I couldn't configure runners.feature_flags, so I added support for feature flags which are described here: https://docs.gitlab.com/runner/configuration/feature-flags.html#available-feature-flags

Other fixes:

• fixed some linting and bool values for consistency
• only add machine section if machine_MachineOptions is set

If I should split this PR into separate ones, please let me know.

[guenhter]

Thx for your contribution