If addresses of dependent services are not set,
the clients will return an error which will be logged
and an error message will be returned to the caller.
This allows the policy service to continue working with
policies which doesn't require the use of additional
TSA services like cache, task, signer and did resolver.

Headers can be accessed with a built-in Rego function
named get_header("header_name") instead of being a
map value inside the input.

This commit changes two things:
* evaluationID is no longer returned as part of the JSON response, but instead is
returned as HTTP ETag header
* if policy source code contains a single blank variable assignment then the result
from the policy evaluation will be only the value of that variable

This is an experiment to produce a generic evaluation
function for different polices, which is able to return
arbitrary JSON result after policy execution. For the
moment a lot of conventions have to be followed in order
for this to work, which is not so good. More experiments
will follow to reduce the conventions to minimum (it would
be best to remove all of them).