feat: add custom shader support for CompositeCanvasRenderer

[hatayama]

Description

Fixes #22

This PR adds support for custom shader override in CompositeCanvasRenderer through ProjectSettings. Users can
now specify an alternative shader to replace the default "UI/CompositeCanvasRenderer" shader used by all
CompositeCanvasRenderer components.

Key Changes:

• Add customShader setting to ProjectSettings
• Allow overriding default UI/CompositeCanvasRenderer shader
• Maintain MaterialRepository caching compatibility

Motivation:
This feature enables users to customize the rendering behavior of CompositeCanvasRenderer by using their own
shaders while preserving all existing functionality and performance optimizations.

Type of change

Please write the commit message in the format corresponding to the change type.
Please see Conventional Commits for more information.

• 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 not work as expected)
• Update documentations
• Others (refactoring, style changes, etc.)

Test environment

• Platform: Editor(Mac)
• Unity version: 2023.2+
• Build options: IL2CPP, .Net Standard 2.1

Checklist

• This pull request is for merging into the develop branch
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have checked my code and corrected any misspellings