Provide access to user secrets from ECL code

Add support for retrieving user secrets from ecl code. There should be a check to ensure that they can be restricted to signed code.
It would be good to try and ensure that they cannot be added to log files etc., but that may prove tricky since we can't really mark them as tainted.

Likely to be more secure though than the alternatives of embedding in ecl code. Worth comparing with storing secrets in logical files.