Accept predicate value raw-string and base64

[fabricereix]

Currently predicates accept primitive literals such as

jsonpath "$.id" == "123"
jsonpath "$.value" == true
sha256 equals hex,a8100ae6aa1940d0b663bb31cd466142ebbdbd5187131b92d93818987832eb88;

We could accept more literals, reusing the one defined for the request/response body.

body == base64, SGVsbG8gV29ybGQh;  
jsonpath "$.content" == ```List of items
- item1
- item2
- item3
```