Rollup node: support "remote:" signer scheme
What
This MR adds support in the rollup node for the remote: signing scheme and the argument --remote-signer.
In addition, the rollup node now ensures that the remote signer is reachable on startup and that it knows all the required keys.
Why
Signing would fail with --remote-signer but only when injecting.
Manually testing the MR
dune exec tezt/tests/main.exe -- --file sc_rollup.ml remote signer
Edited by Alain Mebsout