gitlab_*_variable: support masked-and-hidden variables (#10787)
* gitlab_*_variable: support masked-and-hidden variables
Support masking and hiding GitLab project and group variables. In the
GitLab API, variables that are hidden are also masked by implication.
Note gitlab_instance_variable is unmodified since instance variables
cannot be hidden.
* gitlab_*_variable: add `hidden` to legacy `vars` syntax
* gitlab_*_variable: address review comments in doc
(cherry picked from commit 3574b3fa93)
Co-authored-by: David Phillips <phillid@users.noreply.github.com>
