Review Terraform plans in Terraform related MRs
### Problem to solve
As a DevOps engineer, in order to review `terraform plan` outputs, I'd like to see the `plan` output in a merge request so that I can add comments and discuss changes with fellow DevOps engineers prior to merge.
[Opportunity canvas](https://docs.google.com/document/d/19HFHgsHhL09lewv5VNdqBQXtq0XxT7Kaj8RtTie1_yM/edit)
### Intended users
* [Rachel (Release Manager)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#rachel-release-manager)
* [Devon (DevOps Engineer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#devon-devops-engineer)
* [Sidney (Systems Administrator)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sidney-systems-administrator)
### Proposal
#### MVC steps
1. Provide the summary in a widget (current issue)

- Find a way to measure the usage of this feature following the usage ping and analytics possibilities we have today. We are interested in the number of MRs that receive this widget.
2. Provide the plan output: main parts or full plan (follow-up issue, depends on https://gitlab.com/gitlab-org/ux-research/issues/698)
- Build upon / Extend: https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/ci/templates/Terraform.gitlab-ci.yml
- Use a linter-like output that shows a formatted `terraform plan` output into the MR that started to job.
### Permissions and Security
View of report should be consistent with other merge request result items and should not contain secrets.
### Documentation
An issue was created to document.
### Availability & Testing
Should be tested on an example project to ensure it works.
### What does success look like, and how can we measure that?
Adoption of terraform plan reports in merge requests, measured through a persicope report. (See issue)
## Development
### First Iteration
- [x] Add tfplan conversion script to Terraform.gitlab-ci.yml template https://gitlab.com/gitlab-org/gitlab/issues/207526
- [x] Handle terraform report in gitlab-runner https://gitlab.com/gitlab-org/gitlab-runner/issues/10124
- [x] Handle terraform report in gitlab (rails) https://gitlab.com/gitlab-org/gitlab/issues/207527
- [x] Add terraform report to merge request widget https://gitlab.com/gitlab-org/gitlab/issues/207528
- [x] Update documentation to include instructions on how to get terraform plan results https://gitlab.com/gitlab-org/gitlab/issues/207530
- [x] Add usage ping for terraform reports https://gitlab.com/gitlab-org/gitlab/issues/207532
### Future iterations
- [ ] Create issues based on the results of https://gitlab.com/gitlab-org/ux-research/issues/698
epic