Feat: Importing terraform resources takes too long

[ffforest]

What would you like to be added?

Importing a terraform resource takes around 1 minute for the preview step and about 30 seconds for the apply step.
Kusion currently has a --max-concurrent flag that specifies the number of concurrent resource node executions (including terraform) that defaults to 10. This is still relatively slow. Importing 120 resources takes about 20 minutes to complete (Every 10 resources takes about 1 minute and half).
This issues tracks the effort to reduce the 1 minute and half time frame.

Why is this needed?

Another workaround to this is to up the number of --max-concurrent but that has other consequences.
Each terraform CLI execution allegedly takes ~100MB memory usage so a safe number of --max-concurrent may differ case by case.