Support for setting runtime through netlify_site_build_settings

[SalehAly01]

We noticed an issue while creating Next.js and setting the build setting using the netlify_site_build_settings resource. The app is detected on Netlify as Next.js and builds successfully, but the apps return an error App Not Found while trying to access them.

After debugging the issue, we found that we have to manually change the runtime inside the build and deploy settings section in Netlify UI and then trigger a new deployment to fix it.

It would be great if the resource could support setting the runtime through Terraform so it won't need manual effort with a large number of apps.