Support login_hint in auth code flow

[chlowell]

I see some vestigial code for this:

microsoft-authentication-library-for-go/apps/internal/base/base.go

Lines 190 to 198 in cc87fdb

	// There were left over from an implementation that didn't use any of these. We may
	// need to add them later, but as of now aren't needed.
	/*
	if p.ResponseMode != "" {
	urlParams.Add("response_mode", p.ResponseMode)
	}
	if p.LoginHint != "" {
	urlParams.Add("login_hint", p.LoginHint)
	}

Please add at least login_hint; the Azure SDK would use it.