Add source_uri to container messages

[david4096]

For messages that can be backed directly by a file (BAM for readgroupset, VCF for variants) we could provide a field named source_uri, as has been added to the Reference Set message here .

Adding a source_uri field evenly across the API would make it easy to use the access method that suits one's use case. In the end, a server would present the HTTP protocols for discovering data, and then one would download the file as needed. This could be an optional feature of the protocol, however, I expect that being able to serve files along with JSON will be highly desired https://github.com/ga4gh/server/issues/1542 .