Add c:Phoenix.Endpoint.server_info/1

[mtrudel]

Solves #5553

Paired with mtrudel/bandit#258 over in Bandit-land

@mtrudel will follow with a change to refactor livebook-dev/livebook#2316 once this is merged/released (@josevalim relevant to your recent interests)

[josevalim]

Thank @mtrudel. I am thinking this should be a single function, called server_info(:http | :https). WDYT?

[josevalim]

Perhaps it should be server_info(:http | :https) :: {:ok, address, port} | {:local, path} | {:error, reason}

[mtrudel]

I am thinking this should be a single function, called server_info(:http | :https)

Yep; I'll work that up; won't take a few minutes.

[mtrudel]

Revised and ready to go!

[josevalim]

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