Add new LoginWithOpts

[jdolitsky]

The login function uses a hardcoded "oras" user-agent.

This change is similar to #5 but for login:

opts := []auth.LoginOption{
    auth.WithLoginHostname("example.com"),
    auth.WithLoginUsername("oscar"),
    auth.WithLoginSecret("opponent"),
    auth.WithLoginUserAgent("my-user-agent"),
}
err := dockerauth.LoginWithOpts(opts...)

Resolves oras-project/oras#240

[shizhMSFT]

LGTM