We need a tool to help debug TURN setups (SYN-661)

This issue has been migrated from [#1519](https://github.com/matrix-org/synapse/issues/1519).

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Submitted by @​matthew:matrix.org
General nightmare debugging VoIP between matrix.org & a remote HS because of docker FW problems on their end. Wish we had a script that spelt out whether their TURN was running and talking the right things with the right protocols in the right ports and advertising it correctly in vector etc

(Imported from https://matrix.org/jira/browse/SYN-661)

The issue names no files, tests, or entry points. First define the diagnostic scope for remote TURN connectivity, including firewall behavior, supported protocols and ports, and how the configuration is advertised in Vector; done means a usable tool reports whether each expected check passes.

Written by the indexing model from the issue text.