Add environment specific Infracost diffs to GitHub Actions summery page

[brettcurtis]

As a "workflow approver," I'd like to see all the relevant information on the summary page that would go into my decision-making process when approving the apply job for infrastructure. The cost difference in a given environment would be crucial to that decision.

Acceptance

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1. Can see environment-specific cost differences in the GitHub Actions summary page

Notes

• For the sandbox workflow, we need to compare the cost in the current PR branch to the main branch. This is similar to the old workflows we had before switching to the application.
• We need to compare the cost against the previous commit and main branch for the pre-prod and production environment since pre-prod runs on merge to main and prod runs on successful workflow run of pre-prod.
• Maybe we can utilize the cache for infracost breakdown out files to simplify code vs. dealing with different commits.
• Each environment will have its own tfvars and infracost-usage file
• Ali provide some CLI ideas via email infracost diff --path plan.json --format json --out-file infracost.json and infracost output --path infracost.json --format github-comment --out-file infracost-markdown.md