Support XRootD URLs with multiple servers.

From the TXNetFile docs:

The "url" argument must be of the form

root://server1:port1[,server2:port2,...,serverN:portN]/pathfile,

Note that this means that multiple servers (>= 1) can be specified in the url. The connection will try to connect to the first server:port and if that does not succeed, it will try the second one, and so on until it finds a server that will respond.

I didn't realise this was an pre-existing feature in ROOT but I'd been asking for it for a while. Being able to do this better matches with how data is stored on the grid and makes it easier to when sites are down.

The implementation of this shouldn't only check that the server is alive, it should also fallback if errors are returned when trying to access the data (i.e. only querying the config isn't enough to know if the site is down).

Start by comparing the issue's TXNetFile URL syntax with uproot5's existing remote-file URL handling; the payload does not name a repository file or test. Done means URLs containing multiple server:port entries try each server in order and also fall back after data-access errors, not only after an initial connectivity check.