Add reqadd,rspadd,reqrep,reqirep,rsprep,rspirep to backend template

[stuwil]

Adds missing request and response modification features to the backend template, bringing it to parity with the frontend one.

Breaking change for reqrep on backend: standardized to make this the same format as the frontend template - as in, it should be passed a list.

[benwebber]

Could you add these to the listen template as well?

[stuwil]

@benwebber You've got it!