feat(rust): improve user/root exported information

[imobachgs]

Problem

There are a few problems with the user and root settings that "agama config show" generates.

1. It is not possible to get the root password (it is not exposed). See bsc#1235602.
2. The user section is exported even if a first user does not exist.
3. The root section is exported even if no authentication mechanism has been defined.
4. The hashed_password key is exported even if the root only has an SSH public key.

For instance, here's the output for a just started Agama instance:

{
  "root": {},
  "user": {
    "fullName": "",
    "userName": "",
    "password": "",
    "hashedPassword": false
  }
}

Solution

• Move the password settings to a single struct (UserPassword). The password and the hashedPassword keys are always exported together.
• Do not export the user or root sections unless it is needed.

I decided to not make the changes at HTTP level because it would imply adapting the web client too and we have more urgent things to work on.

Testing

• Updated the unit test
• Tested manually

[coveralls]

coverage: 62.649%. remained the same
when pulling b3b71d0 on export-password
into c19a5b9 on master.

[jreidinger]

this means we still export even empty full_name. Is it correct?

[imobachgs]

Yes, the full_name is not mandatory.

[jreidinger]

so in exported profile will be full_name: "" which looks wrong to me.

[jreidinger]

when I see how tricky it is...maybe it belongs more to FirstUser or FirstUserSettings and having trait like Into?

[imobachgs]

OK, I can change that. At first sight, I preferred to keep this logic just here because it is not needed anywhere else and, for consistency, I would have to implement other conversions too (From<&FirstUser> for FirstUserSettings, From<&RootUser> for RootUserSettings and From<&RootUserSettings> for RootUser).

[jreidinger]

in general approach looks good and it is better then before. Please after merge close mine #2238