Allow configuring the default binding for SAML

The SAML metadata generated by Nuxeo sets the HTTP Redirect binding as default. The binding to use by responses from the IdP can normally be configured manually or by importing this metadata, which can be edited manually if required.
However some scenarios rely on automatic setup processes thus we should allow configuring the default binding through configuration properties.
–
Introduced new configuration property 'nuxeo.saml2.loginBindings' that allows configuring available SAML login bindings.
Default is 'HTTP-Redirect,HTTP-POST'. First binding will be flagged as default in the metadata export.