feat(adduser): add adduser plugin (#10441)

Automatic "Oh My ZSH" installation when adding new users

plugins/adduser/README.md

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+# adduser
+
+This plugin adds support for installing "Oh My ZSH" when adding new users.
+
+To use it, add `adduser` to the plugins array of your `~/.zshrc` file:
+
+```zsh
+plugins=(... adduser)
+```
+
+## Usage
+
+Just run `adduser` as you normally would do and now:
+
+1. The regular `adduser` command will run.
+2. The shell of the new user will switch to zsh
+3. "Oh My zsh will be installed (as if he would have ran `install.sh` himself).
+
+## NOTES
+
+- It is assumed that the last argument will be the username.<br>*(In rare cases people provide the group as last argument)*
+- `useradd` behaviour is not changed.
+
+## Author
+
+[Nikolas Garofil](https://github.com/ngaro)