saml: Implement SAML2AuthManager interface

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>
server: Add SAML related config params in Config

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>
utils: Remove hard coded strings from SAMLUtils

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>
saml: Have the plugin use IDP metadata from URL, get values from Config

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>
server: Add IDP entity ID config param

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>