Add missing validation of logout request functions

[Lunkentuss]

Currently, there are multiple functions implemented that validates SAML logout responses, for example:

//service_provider.go
func (sp *ServiceProvider) ValidateLogoutResponseRequest(req *http.Request) error {...}
func (sp *ServiceProvider) ValidateLogoutResponseForm(postFormData string) error {..}
func (sp *ServiceProvider) ValidateLogoutResponseRedirect(queryParameterData string) error {...}

. However, there are no corresponding functions for validating logout requests. I would suggest to add the following public functions:

//service_provider.go
func (sp *ServiceProvider) ValidateLogoutRequestRequest(req *http.Request) error {...}
func (sp *ServiceProvider) ValidateLogoutRequestForm(postFormData string) error {...}
func (sp *ServiceProvider) ValidateLogoutRequestRedirect(queryParameterData string) error {...}

, in order to support validation of logout requests.

Thanks in advance!