Rewrite users service from ruby to rust

[mchf]

Problem

Current stable version of Agama is mixture of several languages and technologies. Users service was originally written in ruby. As we later on decided to prefer rust for various reasons, it makes sense to rewrite users service to rust.

Testing

• Added a new unit test
• Tested manually

Documentation (TODO)

Look for relevant sections and adjust:

• The description parts of the JSON schema
• Is the CLI affected? See cli.md for a complete overview,
  change the /// comments (rust doc)
  and update the .md with cargo xtask markdown
• https://agama-project.github.io/ (source)
• Run: git ls-files '*.md'

[mchf]

Current state as a picture (curl -k -H @headers.txt -X GET $AGAMA_URL/api/v2/system):

once I publish even set config message there will also appear password (with values {type: hashed} or {type: plain, value: "whatever set by user"}, field next to name or may be slightly different - until it is set to stone, it can vary ;-)

[imobachgs]

The infrastructure of the service looks good, but the logic is wrong. Agama keeps the configuration for the first user (under the user key) and the root (using the root key). There is nothing like a list of users.

Additionally, it does not read the list of users from the underlying system. It just uses its own configuration.

Please, check User and root for further information about the format.

[imobachgs]

I am sorry, but I would expect this struct to match the old UserSettings struct.

[imobachgs]

Why are you reading the information from the underlying system? Agama just holds the configuration for the first user and the root user. But they has nothing to do with the ones from the live system where Agama is running.