feat(stats): pass plan stats to markdown templates

[inkel]

what

• This PR adds an additional struct to the data passed down to the templates in the Markdown renderer with stats about a successful plan: number of resources to add, update, destroy, if it include changes (something that's already covered but I started working on this branch before that change went in) and if there are any resources changed outside of Terraform.

why

• The benefit of passing down this stats won't probably be notable with the default templates as they are, but it could, however, allow users to render a successful plan like the following:

---

The following environments have Terraform changes

environment	add	change	destroy
staging	3	1	0
dev	0	0	5

The following environments have changes outside of Terraform

• prod-01
• prod-02

---

The template code to do something like that could be:

{{ $withChangesOutside := false }}
### The following environments have Terraform changes
| environment | add | change | destroy |
|-|-|-|-|
{{ range $i, $result := range .Results }}
{{- if $result.Stats.ChangesOutside }}{{ $withChangesOutside = true }}{{ end -}}
{{- if $result.Stats.Changes }}
| `{{ $result.ProjectName }}` | {{ $result.Stats.Add }} | {{ $result.Stats.Change }} | {{ $result.Stats.Destroy }} |
{{ end -}}
{{ end }}

{{ if $withChangesOutside }}
### The following environments have changes outside of Terraform
{{ range $i, $result := range .Results }}
- `{{ $result.ProjectName }}`
{{ end }}
{{ end }}

tests

• I've been testing these changes locally and if everything goes according to plan it is likely we will test it thoroughly at Grafana Labs, where we manage 300+ environments with Atlantis.

• Adding tests to the test suite implies changing the default templates, which is not probably something we would like to introduce unless this is first approved/merged.

references

• First mentioned in Feat: Make "No changes" stand out more + add summary of number of (un)changed projects #2069

[inkel]

I've identified the changes in lines 199-211 as something that could be done for the rest of the templates. In fact, I believe this particular function could be refactored further to make it more readable, however, such a change is out of the scope of this PR.

[nitrocode]

Thank you!

[inkel]

Thanks for merging!