feat: [maintenance] new module for maintenance

[cloud-java-bot]

Generated by @blakeli0 via generate_new_client_hermetic_build.yaml

Command used:

python generation/new_client_hermetic_build/add-new-client-config.py add-new-library --api-shortname "maintenance" --name-pretty "Maintenance API" --api-description "provides a centralized view of planned disruptive     maintenance events across supported Google Cloud products. It offers users     visibility into upcoming, ongoing, and completed maintenance, along with     controls to manage certain maintenance activities, such as mainteance     windows, rescheduling, and on-demand updates." --proto-path "google/cloud/maintenance/api/v1" --product-docs "https://cloud.google.com/unified-maintenance/docs/overview"