feat(cpp): add functions related to user management, and initial high-level client

[slbotbm]

Which issue does this PR address?

Closes #
Relates to #2100

Rationale

Works towards completion of cpp client.

What changed?

Adds the following:

• create_user
• get_user
• get_users
• update_user
• delete_user
  and their tests.

Adds high level client that will function as primary API with documentation. The diff looks big since a lot of the public api was changed and aligned to PascalCase. I have not rewritten the tests using the high level client since I would like to do that at once instead of a gradual approach.

Local Execution

• Passed
• Pre-commit hooks ran

AI Usage

If AI tools were used, please answer:

1. Which tools? codex
2. Scope of usage? code generation
3. How did you verify the generated code works correctly? read through it
4. Can you explain every line of the code if asked? yes