Signer: Add a CLI option to allow proof of possession requests

What

Add a CLI option in the signer to allow proof of possession requests. And adding a test for it.

Manually testing the MR

A new test is added in the MR. One can now run:

make && dune exec tezt/tests/main.exe -- --log-file /tmp/log_$(date "+%Hh%M") --verbose --file tezt/tests/signer_test.ml --title 'Alpha: Prove possession of tz4 test'