Expression based filters

[LakiG]

The filter feature has a parameter - size, which validates the length of string/size of array/no of pairs in the object.
At the moment, it supports equality between the "filter" size and the "response" size.

Add support to define expression-based conditions in filter size (and other new parameters in future).

Example. A test case could be to pass only if the response size is greater than 2. The filter size can be defined as -

filter:
  size: size>2