feat: make pr-based promotions work with kargo render

[krancour]

Fixes #1330

Depends on akuity/kargo-render#268

This PR:

• Upgrades us to Kargo Render v0.1.0-rc.38, which has three important, new features:
  • Can take input from a local repository instead of a remote repository
  • Can write output to a local path instead of a branch in a remote repository
  • Does not refuse to manage an existing branch that lacks Kargo Render metadata, as long as the branch is 100% empty
• Fixes a bug in Kargo's existing PR-based promotions where the base branch is assumed to exist
  • We now create it if it doesn't
• Make Kargo Render promotion mechanism behave much more like the Helm and Kustomize-based ones:
  • Points Kargo Render at the path of the already-cloned repo, which will already be sitting on the desired commit

Taken together, these remove the limitation of Kargo Render and PR-based promotions not working together.

[netlify]

✅ Deploy Preview for docs-kargo-akuity-io ready!

Name	Link
🔨 Latest commit	0e2d71a
🔍 Latest deploy log	https://app.netlify.com/sites/docs-kargo-akuity-io/deploys/660212ba0a223800084448da
😎 Deploy Preview	https://deploy-preview-1674.kargo.akuity.io
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[codecov]

Codecov Report

Attention: Patch coverage is 41.66667% with 49 lines in your changes are missing coverage. Please review.

Project coverage is 43.87%. Comparing base (76d6813) to head (0e2d71a).

Files	Patch %	Lines
internal/controller/promotion/pullrequest.go	0.00%	16 Missing ⚠️
internal/kargo-render/render.go	0.00%	12 Missing ⚠️
internal/controller/git/git.go	11.11%	8 Missing ⚠️
internal/controller/promotion/render.go	80.95%	5 Missing and 3 partials ⚠️
internal/controller/promotion/git.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1674      +/-   ##
==========================================
- Coverage   44.10%   43.87%   -0.24%     
==========================================
  Files         202      202              
  Lines       13043    13024      -19     
==========================================
- Hits         5753     5714      -39     
- Misses       7045     7063      +18     
- Partials      245      247       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.