All-pairs testing in Signal Mapping

[Horolsky]

In addition to Zip and Prod clauses, the Signal Mapping model should support a Pairwise clause
that will select all pairwise combinations of test parameters, giving similar behavior to Prod but with reduced resource usage.

See the concept at Wikipedia page.

Note that we are targeting model parameters rather than a test vector as in the original concept.
The task requires creating a specialized JSON iterator first, see zmbt-framework/backends/cxx/include/zmbt/core/json_iter.hpp