Run `terraform apply` on `publish`

[ndellosa95]

Is your feature request related to a problem? Please describe.
Similar to my issue for terraform plan here, it would be great if we could use pants publish to call terraform apply.

Describe the solution you'd like
pants publish calls terraform apply.

Describe alternatives you've considered
pants experimental-deploy also seems like an appropriate goal for this feature.

[lilatomic]

The discussion around experimental-deploy referenced this as an example for why it was needed: Terraform modules can be published to registries. Publishing to a registry also followed the expectations of the other uses of publish.
I think we may also need a separate target type for Terraform deployments to specify deployment-related data (backends, var files).

I think once we think that through it should be fairly easy to wrap terraform apply.

[lilatomic]

experimental-deploy is functional enough with Terraform that I think we can mark this as solved. Do you think there's any core functionality missing?