feat: add commit statistics to the changelog

[ognis1205]

Description

This adds support for computing and exposing aggregated commit statistics:

• Total number of commits
• Duration (in days) between the first and last commit
• Number of conventional commits
• Link references and their counts
• Days since the previous release

The data is accessible in templates using the {{ statistics }} variable.

Motivation and Context

This change is inspired by the cargo-smart-release tool, which provides similar release statistics.
Including commit statistics in the changelog offers better insight into the release scope and development activity.
This change addresses [FEATURE REQUEST #452] and makes the statistics available in a configurable way through templates.

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 72.72727% with 12 lines in your changes missing coverage. Please review.

Project coverage is 42.62%. Comparing base (7148b2d) to head (6a24eef).

Files with missing lines	Patch %	Lines
git-cliff-core/src/release.rs	72.73%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1151      +/-   ##
==========================================
+ Coverage   42.06%   42.62%   +0.56%     
==========================================
  Files          21       21              
  Lines        1983     2025      +42     
==========================================
+ Hits          834      863      +29     
- Misses       1149     1162      +13     

Flag	Coverage Δ
unit-tests	42.62% <72.73%> (+0.56%)	⬆️

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