feat(template): support using PR labels in the GitHub template

[orhun]

Description

This PR makes it possible to use the pull request labels with the GitHub integration:

* {{ commit.github.pr_labels }}

The final context is the following:

"github": {
    "username": "orhun",
    "pr_title": "some things have changed",
    "pr_number": 420,
    "pr_labels": ["test"],
    "is_first_time": false
}

Motivation and Context

See #461

How Has This Been Tested?

Unit tests.

Screenshots / Logs (if applicable)

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have formatted the code with rustfmt.
• I checked the lints with clippy.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (fe9a74e) 41.40% compared to head (ea60467) 41.30%.

Files	Patch %	Lines
git-cliff-core/src/release.rs	76.93%	3 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #467      +/-   ##
==========================================
- Coverage   41.40%   41.30%   -0.10%     
==========================================
  Files          15       15              
  Lines        1029     1034       +5     
==========================================
+ Hits          426      427       +1     
- Misses        603      607       +4     

Flag	Coverage Δ
unit-tests	41.30% <76.93%> (-0.10%)	⬇️

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