Better control over infracost integration

Currently to enable infracost, two things are necessary:

1. add the infracost label to a stack

2. set the INFRACOST_API_KEY environment variable

In order to disable infracost for specific situations (e.g. local preview run or drift detection run), one has to manipulate the INFRACOST_API_KEY environment variable in order to prevent the call from happening.

It would be good to have better control over this, and not have to manipulate the INFRACOST_API_KEY environment variable (which I assume will still make the call to infracost, and Infracost will block it because of the bad API key).

```
[[ "$TF_VAR_spacelift_run_trigger" = "spacelift/drift_detection" || "$TF_VAR_spacelift_local_preview" = "true" ]] && export INFRACOST_API_KEY=disabled && export INFRACOST_WARN_ON_FAILURE=true || true
```

It would be nice to have an INFRACOST_ENABLED environment value that is possible to configure on the fly, which would prevent the calls from actually made and allowing us better control over the infracost integration.