feat(appset): Add target branch support pullrequestgenerator

[Syst3m1cAn0maly]

Closes #13533

Support a target_branch template which retrieves the target branch of the Pull Request when using the PullRequestGenerator for AppSets.
Also add a target_branch_slug analog to branch_slug.

Add a targetBranchFilter to allow filtering of the Pull Request based on the target branch using a regex.
Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• Optional. My organization is added to USERS.md.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.

Please see Contribution FAQs if you have questions about your pull-request.

[codecov]

Codecov Report

Patch coverage: 77.77% and project coverage change: +0.25 🎉

Comparison is base (5662367) 48.98% compared to head (c9668d4) 49.23%.

❗ Current head c9668d4 differs from pull request most recent head 852d54a. Consider uploading reports for the commit 852d54a to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #13535      +/-   ##
==========================================
+ Coverage   48.98%   49.23%   +0.25%     
==========================================
  Files         249      248       -1     
  Lines       43066    42844     -222     
==========================================
  Hits        21094    21094              
+ Misses      19864    19651     -213     
+ Partials     2108     2099       -9     

Impacted Files	Coverage Δ
applicationset/services/pull_request/github.go	23.61% <0.00%> (-0.34%)	⬇️
.../apis/application/v1alpha1/applicationset_types.go	28.57% <ø> (ø)
applicationset/services/pull_request/utils.go	93.33% <62.50%> (-6.67%)	⬇️
applicationset/generators/pull_request.go	52.94% <100.00%> (+0.70%)	⬆️
...ationset/services/pull_request/bitbucket_server.go	100.00% <100.00%> (ø)
applicationset/services/pull_request/gitea.go	68.75% <100.00%> (+0.66%)	⬆️
applicationset/services/pull_request/gitlab.go	87.50% <100.00%> (+0.22%)	⬆️

... and 21 files with indirect coverage changes

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[crenshaw-dev]

Thanks, @Syst3m1cAn0maly!