feat(config): Support clientlib metadata generation for dynamically created CSS files

[Hugoer]

This PR adds support for generating .content.xml and css.txt files for CSS files created dynamically during the build process (e.g., via PostCSS plugins).
These files are essential for enabling AEM to recognize and serve the generated clientlibs correctly.

Description

• Introduced a new extraEntries configuration option in PostCSS processing to allow metadata generation for target files.
• Supports specifying:

extraEntries: {
  extension: 'css',
  cwd: destinationPath,
  filenamePattern: 'clientlibs-*',
}

• Ensures metadata is generated for CSS files matching the given pattern in the specified output directory.

Related Issue

Fixes #126

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)

Checklist:

• My code follows the code style of this project.
• I have read the CONTRIBUTING document.
• All new and existing tests passed.