Added proxy support

[maikebing]

    [Test]
    public async Task ShouldWorkWithoutToken()
    {
        // https://gitlab.com/gitlab-org/gitlab
        const long GitLabProjectId = 278964;

        var client = new GitLabClient("https://gitlab.com");
        client.Options.Proxy = new System.Net.WebProxy("http://127.0.0.1:7890");
        var project = await client.Projects.GetByIdAsync(GitLabProjectId, new Models.SingleProjectQuery { Statistics = false});

        Assert.That(project, Is.Not.Null);
        Assert.That(project.Id, Is.EqualTo(GitLabProjectId));
        Assert.That(project.Name, Is.EqualTo("GitLab"));
    }

[Toa741]

Hello @maikebing,
Thanks for you're contribution :).
Please check the comments above so I could execute the workflow to validate it.

[maikebing]

Hi, @Toa741 ,I'm done. please validate it.