Update user datasource: allow searching by id and add attributes

[jorcau]

Hi,

This PR updates the user datasource.

It allows searching by id:

data "gitlab_user" "foo" {
  user_id = "2387"
}

It also adds more attributes and better handles the search parameters to avoid using multiple ones at the same time.

Note: the search by id parameter is called user_id because id is a Terraform reserved keyword. Same goes for the provider attribute renamed into user_provider. I'm willing to change this if you have better suggestions.

Best,

[roidelapluie]

Thanks!