Use multiple and per-user `DistributionListUrl`s

[conioh]

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.

[github-actions]

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[conioh]

/feature

[github-actions]

Diagnostic information

Found '/feature', adding tag 'feature'