Make `value` optional when updating CI/CD variables. Variables marked as
`masked_and_hidden` have their value hidden by GitLab and cannot be
retrieved, so `value` must not be required for updates.

Add `masked_and_hidden` to optional create attributes for group and
project variables (admin variables do not support this attribute).
 Also expand the optional attributes for all three variable managers
(admin, group, project) to include `description`, `raw`, and
`environment_scope` where applicable, aligning with the GitLab API.

Add functional tests covering create, get, update, and delete of
masked-and-hidden variables for both group and project scopes.

…roject level

…I actions

Add GroupServiceAccountAccessTokenManager, ProjectServiceAccountAccessTokenManager,
GroupServiceAccountAccessToken, and ProjectServiceAccountAccessToken to the
cli.register_custom_action cls_names in RotateMixin and ObjectRotateMixin.