Add neon.safekeeper_connstrings GUC

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>

docker-compose/compute_wrapper/var/db/postgres/configs/config.json

@@ -104,6 +104,11 @@
                     "value": "safekeeper1:5454,safekeeper2:5454,safekeeper3:5454",
                     "vartype": "string"
                 },
+                {
+                    "name": "neon.safekeeper_connstrings",
+                    "value": "host=safekeeper1 port=5454,host=safekeeper2 port=5454,host=safekeeper3 port=5454",
+                    "vartype": "string"
+                },
                 {
                     "name": "neon.timeline_id",
                     "value": "TIMELINE_ID",