Add method to set Sudo header

[DrMegavolt]

Api doc says I can execute requests on behalf of another user by setting header Sudo

This PR adds such possibility

Usage

	client := gitea.NewClient(os.Getenv("GITEA_HOST"), os.Getenv("GITEA_TOKEN"))
	client.SetSudo(username) // this is new 
	userRepos, err := client.ListMyRepos()

If I have GITEA_TOKEN as Admin User but results will contain repos of username

For reference the test in Gitea that checks if Sudo works
https://github.com/go-gitea/gitea/blob/7096085f2b07246315e95e394b180ce9729efbb0/integrations/api_admin_test.go#L81