Add IPv6 support for Directories (still incomplete)

[softins]

Short description of changes

When complete, this will allow servers with IPv6 addresses to register with a directory using both their IPv4 address and IPv6 address. It should also allow for a server that only has an IPv6 address to register, even though it will be inaccessible to clients that just have IPv4.

CHANGELOG: Server: Add IPv6 support to Directory operations.

Context: Fixes an issue?

No issue, but a long-standing discussion at #1950

Does this change need documentation? What needs to be documented and how?

It will do.

Status of this Pull Request

Incomplete work in progress, posted for visibility and comments

What is missing until this pull request can be merged?

To be completed

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
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