Support endpoints in addition to Docker images

[MikeBishop]

Some implementations may not be able to publish a Docker container for various reasons, but are able to host a publicly accessible server instance. It would be nice to be able to supply a hostname/port as an alternative.

[marten-seemann]

That would indeed be nice, but there are a few problems with that:

1. It only works for servers, not for clients.
2. We use environment variables to configure endpoint behavior.
3. For servers, we give them a directory of files to serve before each test case, and after the download we do a byte-by-byte comparison of what the client downloaded.
4. Performance measurements over the public internet would be hard to compare.

Any idea how to work around these?