fix(pr): multiple bugs in the PR controller

[corrieriluca]

Breaking changes

• PR controller: delete linked layers in discovery just before creating new ones
  • Previously new layers were created (with GenerateName), thus when a PR was updated outdated layers were still planned, resulting in confusing results in the comment.
  • Another strategy would have been to update the existing layers (this was implemented but never done because of the GenerateName uniqueness), but you also have to provide a value to metadata.resourceVersion when doing so, delete and re-create is simpler IMO.

Minor Changes

• When handling push events, only inspect the layers linked to the affected repositories
• Add a few more log entries on PR creation or resource reconciliation to improve log reading
• Fix PR comment template: Markdown headings were not displayed properly when multiple layers were planned
• Add a Planning state to the TerraformPullRequest resource for better UX

[codecov-commenter]

Codecov Report

Merging #187 (cdb221f) into main (983ad07) will increase coverage by 0.15%. The diff coverage is 63.41%.

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@@            Coverage Diff             @@
##             main     #187      +/-   ##
==========================================
+ Coverage   68.93%   69.09%   +0.15%     
==========================================
  Files          28       28              
  Lines        2025     2035      +10     
==========================================
+ Hits         1396     1406      +10     
  Misses        545      545              
  Partials       84       84              

Files	Coverage Δ
internal/controllers/terraformlayer/controller.go	38.02% <100.00%> (ø)
...nal/controllers/terraformpullrequest/controller.go	33.87% <100.00%> (-1.05%)	:arrow_down:
internal/controllers/terraformrun/controller.go	62.62% <100.00%> (ø)
internal/webhook/event/pullrequest.go	82.19% <100.00%> (+0.24%)	:arrow_up:
internal/webhook/event/push.go	55.84% <100.00%> (+1.79%)	:arrow_up:
...nal/controllers/terraformpullrequest/conditions.go	85.83% <0.00%> (+3.25%)	:arrow_up:
internal/controllers/terraformpullrequest/layer.go	80.43% <66.66%> (-3.57%)	:arrow_down:
...nternal/controllers/terraformpullrequest/states.go	57.95% <20.00%> (+0.97%)	:arrow_up:

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