feat(gooddata-sdk): add CatalogAppearanceService for themes and color palettes

[hkad98]

Summary

• Add CatalogAppearanceService exposing theme and color palette CRUD via sdk.catalog_appearance
• Add entity models: CatalogTheme, CatalogThemeAttributes, CatalogColorPalette, CatalogColorPaletteAttributes
• Wire AppearanceApi through GoodDataApiClient and expose the service on GoodDataSdk
• Add documentation for all 10 methods (list/get/create/update/delete for both entities)

The GoodData API already exposed these endpoints via AppearanceApi, but the SDK service layer had no wrapper — users had to drop down to the raw API client. This follows the existing service-based facade pattern (matching CatalogOrganizationService).

[codecov]

Codecov Report

❌ Patch coverage is 65.78947% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.36%. Comparing base (12e1e2d) to head (0dc9bf0).

Files with missing lines	Patch %	Lines
...sdk/src/gooddata_sdk/catalog/appearance/service.py	42.85%	32 Missing ⚠️
...k/catalog/appearance/entity_model/color_palette.py	86.95%	3 Missing ⚠️
...ddata_sdk/catalog/appearance/entity_model/theme.py	86.95%	3 Missing ⚠️
packages/gooddata-sdk/src/gooddata_sdk/sdk.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1412      +/-   ##
==========================================
- Coverage   77.45%   77.36%   -0.09%     
==========================================
  Files         225      228       +3     
  Lines       14661    14775     +114     
==========================================
+ Hits        11355    11430      +75     
- Misses       3306     3345      +39     

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