ssh: made the ssh config file path modular

[Neon-44]

made a new option programs.ssh.userConfigFile with default ".ssh/config" and replaced home.file.".ssh/config".text with home.file.${cfg.userConfigFile}.text

Description

adds an Option programs.ssh.userConfigFile which expects a String returned (default .ssh/config).
replaces home.file.".ssh/config".txt with home.file.${cfg.userConfigFile}.txt
can be used together with an Alias (ssh = ssh -F /path/to/file) to declutter the home directory
default option for userConfigFile is still ".ssh/config"

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

this PR doesn't add a new Module.

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[nechtarg]

Shouldn't the generation of the alias be another option included with this?

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[nechtarg]

Okay, I think that can be put aside for another pr. Having this here merged would be huge

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[Aleksanaa]

Still important

[teto]

rather than adding yet another config item, you could disable the writing of the file (e.g. xdg.configFile."nix/nix.conf".enable = false;) and write its output somewhere else. I am not absolutely sure you can copy the content of the file but worth a try, it should still be available even when disabled.
`

[Neon-44]

the wording @khaneliman suggested is more intuitive, let's go with that instead ^^