Plumb through both libpq and grpc connection strings to the compute

Add a new 'pageserver_connection_info' field in the compute spec. It
replaces the old 'pageserver_connstring' field with a more complicated
struct that includes both libpq and grpc URLs, for each shard (or only
one of the the URLs, depending on the configuration). It also includes
a flag suggesting which one to use; compute_ctl now uses it to decide
which protocol to use for the basebackup.

This is compatible with everything that's in production, because the
control plane never used the 'pageserver_connstring' field. That was
added a long time ago with the idea that it would replace the code
that digs the 'neon.pageserver_connstring' GUC from the list of
Postgres settings, but we never got around to do that in the control
plane. Hence, it was only used with neon_local. But the plan now is to
pass the 'pageserver_connection_info' from the control plane, and once
that's fully deployed everywhere, the code to parse
'neon.pageserver_connstring' in compute_ctl can be removed.

The 'grpc' flag on an endpoint in endpoint config is now more of a
suggestion. Compute_ctl gets both URLs, so it can choose to use libpq
or grpc as it wishes. It currently always obeys the 'prefer_grpc' flag
that's part of the connection info though. Postgres however uses grpc
iff the new rust-based communicator is enabled.

TODO/plan for the control plane:

- Start to pass `pageserver_connection_info` in the spec file.
- Also keep the current `neon.pageserver_connstring` setting for now,
  for backwards compatibility with old computes

After that, the `pageserver_connection_info.prefer_grpc` flag in the
spec file can be used to control whether compute_ctl uses grpc or
libpq.  The actual compute's grpc usage will be controlled by the
`neon.enable_new_communicator` GUC. It can be set separately from
'prefer_grpc'.

Later:

- Once all old computes are gone, remove the code to pass
  `neon.pageserver_connstring`

test_runner/fixtures/neon_fixtures.py

@@ -4258,7 +4258,13 @@ class Endpoint(PgProtocol, LogUtils):
         # If gRPC is enabled, use the new communicator too.
         #
         # NB: the communicator is enabled by default, so force it to false otherwise.
-        config_lines += [f"neon.enable_new_communicator={str(grpc).lower()}"]
+        #
+        # XXX: By checking for None, we enable the new communicator for all tests
+        # by default
+        if grpc or grpc is None:
+            config_lines += [f"neon.enable_new_communicator=on"]
+        else:
+            config_lines += [f"neon.enable_new_communicator=off"]
 
         # Delete file cache if it exists (and we're recreating the endpoint)
         if USE_LFC: