Use multiple and per-user `DistributionListUrl`s

Nobody has claimed this yet.

**Is your feature request related to a problem? Please describe.**

1. I want to use additional distribution sources.
I am not an admin.
Everything else I do with WSL doesn't require administrative privileges and doesn't affect global state and suddenly this does.

2. I want to use multiple additional distribution sources.
I can't.
I can either replace the Microsoft official source or add a single additional source.

**Describe the solution you'd like**

1. I want WSL to consider `HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Lxss` and not only the same path under `HKEY_LOCAL_MACHINE`.

2. `DistributionListUrlAppend` should be `REG_MULTI_SZ` and WSL should consider the multiple sources that may appear there.

**Describe alternatives you've considered**

For the first issue - there aren't any.
For the second issue, a workaround might be to create a local server that generates a combined JSON on the fly.

**Additional context**

See also #13099 for another improvement request regarding the distribution lists.

Start by locating the WSL code that reads DistributionListUrlAppend and the Lxss registry settings; the issue names the HKCU and HKLM paths but no files or tests. Confirm how per-user settings and multiple REG_MULTI_SZ sources should be combined, then verify behavior for both administrative and non-administrative users.

Written by the indexing model from the issue text.