Add neon.safekeeper_conninfo_options GUC (#11901)

In order to enable TLS connections between computes and safekeepers, we
need to provide the control plane with a way to configure the various
libpq keyword parameters, sslmode and sslrootcert. neon.safekeepers is a
comma separated list of safekeepers formatted as host:port, so isn't
available for extension in the same way that neon.pageserver_connstring
is. This could be remedied in a future PR.

Part-of: https://github.com/neondatabase/cloud/issues/25823
Link:
https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-PARAMKEYWORDS

Signed-off-by: Tristan Partin <tristan@neon.tech>

safekeeper/tests/walproposer_sim/simulation.rs

@@ -87,6 +87,7 @@ impl WalProposer {
         let config = Config {
             ttid,
             safekeepers_list: addrs,
+            safekeeper_conninfo_options: String::new(),
             safekeeper_reconnect_timeout: 1000,
             safekeeper_connection_timeout: 5000,
             sync_safekeepers,