Expression based filters

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
```

Start by locating the filter-size validation and its existing equality handling; the payload does not name specific files or tests. Define the expression syntax and evaluation boundary first, then verify that a filter using `size: size>2` passes only when the response size is greater than 2.

Written by the indexing model from the issue text.