feat(terraform-pipeline.yml): enable git tag pipelines and targeted Terraform actions

This merge request implements the following for the terraform-pipeline.yml template:

• Allow jobs to run on git tag actions. This was missed when the template was originally written as teams were not using git tags to managed the infrastructure repository versioning. However, this is now used so needs to be supported.
• Allow targeted Terraform plan actions. This is not to be used very often, however, to support a requirement to be able to trigger a targeted deployment of a specific webapp container from the upstream webapp repo we need to implement this ability.

Here is an example pipeline job using the targeting variable - https://gitlab.developers.cam.ac.uk/uis/devops/experiments/webapp-demo/infrastructure/-/jobs/1229256

Here is an example pipeline running as normal without targeting - https://gitlab.developers.cam.ac.uk/uis/devops/experiments/webapp-demo/infrastructure/-/pipelines/433985