Add support for hidden field to project variables

[yogeshlonkar]

GitLab has inconsistent naming of the filed for hidden variables.
It seems for keeping UI consistant, GitLab chose to introduce different
naming for the same field. This could have been easy handled in UI by
setting two variables on API request instead of introducing combined
field for UI.

I have added 2 test cases for create and update project API as per the
gitlab.com API that I manually tested and added similar unit tests.

[timofurrer]

@yogeshlonkar thanks for the contribution 🤝 I've left some clarification questions on the correctness of the field / field naming. Back to you 🏓

[RicePatrick]

I went ahead and tested this against a locally running GitLab with the 2 suggested changes, and the tests pass. If you'd please make those changes, I'm good to merge.

[RicePatrick]

Here is the test I used to test the above 2 suggestions:

func TestProjectVariablesService_MaskAndHidden_RealGitLab(t *testing.T) {
	client, err := NewClient("<token>",
		WithBaseURL("http://localhost:8085"),
	)
	if err != nil {
		t.Fatalf("Failed to create client: %v", err)
	}
	// create a project
	project, _, err := client.Projects.CreateProject(&CreateProjectOptions{
		Name: Ptr(fmt.Sprintf("test-%d", time.Now().UnixMicro())),
	})
	if err != nil {
		t.Fatalf("Failed to create project: %v", err)
	}
	projectVar, _, err := client.ProjectVariables.CreateVariable(project.ID, &CreateProjectVariableOptions{
		Key:             Ptr("test"),
		Value:           Ptr("testtesttest"), // must have a min of 8 characters
		MaskedAndHidden: Ptr(true),
	})
	if err != nil {
		t.Fatalf("Failed to create project variable: %v", err)
	}

	assert.True(t, projectVar.Hidden)
}

[yogeshlonkar]

@RicePatrick @timofurrer can you please review PR again

[RicePatrick]

Updated code looks good; merging so it's available for the migration tomorrow :)

Changes made as part of further commits