Enable supporting multiple bridge authorities

BridgeDB already supports multiple bridge authorities. To add an additional bridge authority, it needs to be added to the bridgedb config in the BRIDGE_AUTHORITY_DIRECTORIES list.

A bridge should be able to select a bridge authority from the list of authorities, where multiple bridge authorities can be represented, and try one at a time until it is able to successfully upload its descriptor.

Note: This feature might already exist in tor, so auditing the code to check first would be useful.