Add v2 updateinfo API endpoint with product slug support

Implements a new updateinfo.xml API endpoint that supports:
- Product slug-based routing (rocky-linux, rocky-linux-sig-cloud)
- Major version aggregation across all minor versions
- Required architecture filtering to prevent cross-contamination
- Data integrity validation with supported_product_id checks
- Shared XML generation function for code reuse

Key improvements over v1:
- Prevents cross-product package contamination via explicit FK filtering
- Cleaner URL structure using product slugs
- Better error handling and validation
- Comprehensive unit test coverage

Changes:
- apollo/server/routes/api_updateinfo.py: Add v2 endpoint and shared XML generation
- apollo/tests/test_api_updateinfo.py: Add unit tests with mock database
- apollo/tests/BUILD.bazel: Register new test target

Author: rockythorn