Link checker should check .xml files as well

.NET uses .xml files for documentation, and we recently discovered some have broken (404) links. We should add .xml files to the link checker as well or, if too noisy for all language repos, make the file pattern list configurable per-repo.

See Azure/azure-sdk-for-net#36066 for context.

No implementation file, test, or entry point is named. Start by locating the link checker's file-pattern list and any repository-specific configuration, then inspect how language repositories are selected. Done means XML documentation links are checked without unacceptable noise, or the file patterns can be configured per repository.

Written by the indexing model from the issue text.