Add API to allow for adding baseURL to list of URLS parsed from manifest

[dsparacio]

Certain encoders do not allow or support the alt base URL. Use case for Primary to Backup failover. It would be great if a player dev could add a base url manually in this case.

[dsilhavy]

With the new content steering feature and PATHWAY-CLONES it is possible to add new BaseURLs with a simple server-side implementation.