Add option for disabling STS client WS-MEX invocation using WSA EPR address

Currently the STS client parses the contents of Issuer element in endpoint contract to find out the address of STS.
The contract can actually include an EPR with address and optionally some WS-MEX metadata the client could use to send a WS-MEX call to the STS.
If the MEX metadata are not available, the client can still send a WS-MEX call to the address specified in the EPR WSA address, which could however result in simply logging error messages (if the STS does not support WS-MEX, for instance).
This jira is about offering an option for disabling such "aggressive" WS-MEX invocation and avoiding using WS-MEX if the endpoint contract does not mention the STS support that.