feat: add litellm user api

[Gilles-Marco]

Dependency

PR dependent of #6

Changes

Added the feature to create/update/delete/read litellm user

Testing

Here is the terraform test resource I used

resource "litellm_user" "example_user" {
    user_id = "[email protected]"
    user_alias = "[email protected]"
    user_email = "[email protected]"
    user_role = "proxy_admin"
    metadata = {
      "test-metadata" = "test"
    }
    max_budget = 10.2
    rpm_limit = 15
    tpm_limit = 10
    auto_create_key = false
    budget_duration = "30d"
    models = ["azure/gpt-4o"]
    send_invite_mail = false
}

Why no teams

Same reason #7
I don't think it's a good idea for the provider to be able to register an user in a team by multiple ways. It adds a lot of complexity for the provider maintainers, and forcing maintainers to make an opinionated decision about how to do it.
The simplest way for me and unopinionated one, is to have a resource dedicated to it. It doesn't add a lot of complexity for terraform users that way.

Team membership resource will be added on the future, it is actually the next resource I will be providing.