refactor: initialize config structs with default values

[Cyclonit]

Description

This change uses the embedded configuration to initialize configuration options with default values whenever possible. This moves the concern of defining default values into the embedded config and reduces complexity when using the configuration.

Motivation and Context

Implements #1087.

How Has This Been Tested?

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: Patch coverage is 48.71795% with 20 lines in your changes missing coverage. Please review.

Project coverage is 41.82%. Comparing base (bf50336) to head (27896eb).

Files with missing lines	Patch %	Lines
git-cliff/src/lib.rs	0.00%	19 Missing ⚠️
git-cliff-core/src/changelog.rs	83.34%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1090      +/-   ##
==========================================
+ Coverage   41.42%   41.82%   +0.41%     
==========================================
  Files          21       21              
  Lines        1811     1803       -8     
==========================================
+ Hits          750      754       +4     
+ Misses       1061     1049      -12     

Flag	Coverage Δ
unit-tests	41.82% <48.72%> (+0.41%)	⬆️

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