[pick_first] changes to support dualstack design (grpc#34218)

This rolls forward only the pick_first changes from grpc#32692, which were
rolled back in grpc#33718. Specifically:
- Changes PF to use its own subchannel list implementation instead of
using the subchannel_list library, since the latter will be going away
with the dualstack changes.
- As a result of no longer using the subchannel_list library, PF no
longer needs to set the `GRPC_ARG_INHIBIT_HEALTH_CHECKING` channel arg.
- Adds an option to start a health watch on the chosen subchannel, to be
used in the future when pick_first is the child of a petiole policy.
(Currently, this code is not actually called anywhere.)

src/core/BUILD

@@ -4789,15 +4789,15 @@ grpc_cc_library(
     language = "c++",
     deps = [
         "channel_args",
-        "grpc_lb_subchannel_list",
         "grpc_outlier_detection_header",
+        "health_check_client",
+        "iomgr_fwd",
         "json",
         "json_args",
         "json_object_loader",
         "lb_policy",
         "lb_policy_factory",
         "subchannel_interface",
-        "//:channel_arg_names",
         "//:config",
         "//:debug_location",
         "//:gpr",
@@ -4806,7 +4806,6 @@ grpc_cc_library(
         "//:orphanable",
         "//:ref_counted_ptr",
         "//:server_address",
-        "//:work_serializer",
     ],
 )