feat(config): add changelog.render_always option

[orhun]

Description

This PR adds a new option to the configuration file as follows:

[changelog]
render_always = true

The reasoning behind this is that some users might want to always render the changelog, even if there are no changes.

Implemented as a config option to avoid breaking changes.

Motivation and Context

closes #802

How Has This Been Tested?

Test fixtures / locally.

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 9.09091% with 10 lines in your changes missing coverage. Please review.

Project coverage is 39.96%. Comparing base (fbb643b) to head (5f64bc6).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
git-cliff/src/lib.rs	0.00%	6 Missing ⚠️
git-cliff-core/src/changelog.rs	20.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #859      +/-   ##
==========================================
- Coverage   40.15%   39.96%   -0.19%     
==========================================
  Files          21       21              
  Lines        1669     1677       +8     
==========================================
  Hits          670      670              
- Misses        999     1007       +8     

Flag	Coverage Δ
unit-tests	39.96% <9.10%> (-0.19%)	⬇️

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