This GUC is very similar to neon.safekeepers, but instead of being
formatted as host:port, it is a Postgres connection string. The purpose
for changing how we connect to safekeepers is so that we can pass
various libpq SSL keyword parameters in the connection string.
A future PR will remove the `neon.safekeepers` GUC.
Signed-off-by: Tristan Partin <tristan@neon.tech>