Add -domain-config-source config flag for Pages

Follow-up issue on enabling different source configuration for Pages based on #210010 (comment 342953588). Tracking changes for Pages, Documentation and Omnibus on this issue.

Add -domain-config-source flag to Pages

1. Add -domain-config-source to Pages with possible values auto,disk,gitlab but do nothing. gitlab-pages!284 (merged)
2. Implement polling for /internal/pages/status -> Poll every N seconds for M retries.
3. Implement -domain-config-source=disk -> start polling API status in the background -> enable API for serverless domains -> enable disk source by default.
4. Add flag -domain-config-source to Omnibus but default to nil
5. Update documentation with -domain-config-source=disk usage !29103 (closed) could be used as reference

/cc @jmeshell @sean_carroll @vshushlin