Support overriding KeyStore alias for signature so that it can be different than user name used for UsernameToken

Currently, when signing a message, the KeyStore alias lookup is performed using the user name from userInfo (which is set in SignatureAction and comes from request data).

This way, the alias in the KeyStore cannot be different from the user name used for UsernameToken authentication.

Some usage scenarios cannot make such an assumption.

E.g. a common configuration is to prompt the user for the username and password, but the KeyStore is distributed with the client application and contains a static entry with a static password for the signing keypair and certificate, and will be used by multiple users (the WS signature comes from the client application, not an individual user). The KeyStore, and signing certificate alias and password is part of application's configuration.

The password for UsernameToken can be differentiated using a proper password callback handler (since the callback it receives specifies in the "usage" property what is the password needed for - e.g. WSPasswordCallback.USERNAME_TOKEN or WSPasswordCallback.SIGNATURE).

A user found a workaround for this problem for Apache Axis:
http://www.nabble.com/Signature-Alias-vs.-Username-Token-User-td21334511.html

However, there's no simple method for differentiating the user name used by the Signature and UsernameToken actions if WSS4J is not used from Axis, but e.g. CXF.

I've implemented a simple solution by introducing a new handler configuration property - SIG_KEYSTORE_ALIAS - which allows to override the KeyStore alias for the Signature action.