Allow PROXY_PROTOCOL to be per-port customizable.

[alex4108]

What type of PR?

Enhancement

What does this PR do?

Allow PROXY_PROTOCOL env variable to be set on only a specific port. Eg, in my case, only tcp/25 needs to have PROXY_PROTOCOL enabled.

Prerequisites

Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.

• [ x ] In case of feature or enhancement: documentation updated accordingly
• Unless it's docs or a minor change: add changelog entry file.

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[nextgens]

I don't think this is something we want (in general): without documentation it's worthless and making good documentation about it is complex.

I also fail to see why we wouldn't go the extra mile and allow PROXY_PROTOCOL to be a comma separated list of ports we'd allow if we go down that route.

[nextgens]

Suggested change

	* ``smtp`` to accept the ``PROXY`` protocol on TCP 995 only
	* ``pop3s`` to accept the ``PROXY`` protocol on TCP 995 only

[nextgens]

#3060 was asking for something similar

[mergify]

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