feat/Add an option to use placeholders (${index} and ${mem}) in the payload.

[idanmiara]

Sometimes we need the payload to be unique for each iteration.
This PR adds the option to embed mem and/or iteration index into the payload using placeholders.

If you like this idea I'll try to add a test for it (If you can point me to some reference, like maybe a related feature PR with a relevant test or some other ref, that might be helpful)