FTS3 backend: pass activity attribute in copyjob

[nikmagini]

In FTS3, each file in the json copyjob can have an "activity" attribute that can be used for monitoring, and to set activity shares in the FTS3 server:

http://fts3-docs.web.cern.ch/fts3-docs/fts-rest/docs/bulk.html

In the initial implementation, we could set "PhEDEx/priority" (e.g. "PhEDEx/low") as 'activity' attribute for each file.

We could later extend the activity tags to support more detailed values, including e.g. group, campaign - but this requires changes to the schema and central agents to propagate such information down to the T_XFER_TASK table and the FileDownload agent.