Add packet-test protocol server

[robertodauria]

Not meant to be merged.

This is an example of how to serve a third protocol from msak-server, in case we ever need it and also as an example for future integrations. Tested with:
go run ./cmd/msak-server -pt.enabled

And the packet-test client at https://github.com/m-lab/packet-test/blob/main/cmd/client/ndt7/ndt7.go (changing the port to 8081).

Notes:

• I spawn a second HTTP server due to the fact the netx in this repository and netx in ndt-server are two different netxs and so that the PT server has the Conn it expects (I suspect this highlights the need to merge the two and perhaps move netx to m-lab/go)
• The path in the pt Go client appears to be wrong, I had to change it to the value of static.NDT7DownloadURLPath so it could work

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