Create a SAML javascript mapper

[AlistairDoswald]

The idea is to create a generic mapper capable of creating attributes from a javascript description. These mappers could then be used to create values from any value present in the context. Such a mapper may even be able to call an external API to get such information.

The idea is the following: create a mapper similar to the org.keycloak.protocol.saml.mappers.UserAttributeStatementMapper (or corresponding com.quest.keycloak.protocol.wsfed.mappers.SAMLUserAttributeStatementMapper), however in the transformAttribute evaluate a javascript code to obtain the attribute rather then obtaining it from a user's attributes.

Such a use of javascript in keycloak is already done, for example, for the Authorization module, with the javascript-based policy.

[AlistairDoswald]

Added the SAMLScriptBasedMapper for this functionality.