Add config option turn_shared_secret_path

[V02460]

Add the config option turn_shared_secret_path and accompanying docs. The config allows Synapse to read the TURN shared secret from a file. The code was shamelessly adapted from the implementation of registration_shared_secret_path.

Reading secrets from files has the security advantage of separating the secrets from the config. It also simplifies secrets management in Kubernetes.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct
  (run the linters)

[anoadragon453]

Looks good to me, thanks!

(This will also be useful to NixOS users 😊)